You go to bed on a clear night, a storm rolls through Sydney overnight, and by morning your pool has changed colour. Sometimes it’s green. Sometimes it’s brown or murky. Sometimes it starts brown and turns green a day or two later. All of these are common calls I get as a pool cleaner on the Northern Beaches — and each one tells a different story about what the rain actually did to your pool.

Green and brown are caused by different things and need different treatment. Using the wrong approach wastes time and money, and can make the problem worse. Understanding which one you’re dealing with — and why — is the first step to fixing it fast.

Why rain makes your pool go green

Rain doesn’t cause algae by itself — algae spores are always present in pool water at low levels. What rain does is give those spores everything they need to take over. Three things happen simultaneously when heavy rain hits a Sydney pool:

1. Chlorine gets diluted. Every litre of rainwater that enters your pool reduces the concentration of chlorine in the water. A pool with 50,000 litres that takes on 2,000 litres of rain has lost about 4% of its chemical concentration immediately. In a heavy overnight storm with runoff flowing in from garden beds and paths, the dilution is far more significant. When free chlorine drops below 1 ppm, algae can grow unchecked.

2. pH drops fast. Rainwater is naturally acidic — typically around pH 5.5 to 6.0 on Sydney’s coast, compared to the 7.2–7.6 your pool needs. When acidic rain mixes with your pool water, it drives the pH down. This is the part most people miss: even if you have chlorine in the water, chlorine becomes significantly less effective when pH drops below 7.2. At pH 6.8, chlorine is working at about a quarter of its normal efficiency. You can have 2 ppm of free chlorine and still watch your pool turn green if the pH is wrong.

3. The pool gets flooded with algae fuel. Storm runoff carries phosphates, nitrogen compounds, organic matter, soil and algae spores from your garden, lawn, driveway and roof into the pool. Phosphates are algae’s preferred fertiliser — even a small amount can trigger rapid growth. A single heavy storm can deposit more algae nutrients into your pool than weeks of normal use.

These three factors — low chlorine, wrong pH, high nutrients — hit at the same time. That’s why pools that went green after rain can look perfectly fine before a storm and visibly green within 24 hours.

On the Northern Beaches specifically: coastal storms are often intense and fast-moving — 40–60mm in a few hours is not unusual in summer and through the June–August wet months. Pools on sloped blocks, or with gardens that drain toward the pool area, can take on an enormous runoff load in a single event. The pool owner in Collaroy and the one in Balgowlah Heights are both dealing with the same chemistry, but the Balgowlah pool on a steep block with a native garden above it is likely getting far more runoff than a flat coastal block.

Why your pool might go brown instead

A brown or murky pool after rain is a different problem entirely. It’s not algae — it’s physical turbidity. Stormwater runoff from garden beds, lawns, driveways and paths carries suspended sediment, mud, fine soil particles and organic matter directly into the pool. The water goes brown or cloudy brown because it’s essentially dirty — full of particles too fine to settle on their own.

This is exactly what the before photo at the top of this post shows: a lap pool on the Northern Beaches after a heavy overnight storm sent a surge of muddy stormwater across the garden and into the pool. The water is brown and opaque — not from algae, but from suspended soil and organic matter. Phosphates and nitrogen from the garden were now dissolved in the water too, ready to feed algae once the chemistry settled.

The distinction matters for treatment. A brown pool doesn’t need a shock dose of chlorine first — it needs the suspended particles removed before you can do anything useful with chemistry. Shocking a brown pool just burns through chlorine trying to oxidise organic matter without clearing the water.

Sometimes both happen in sequence: the pool goes brown immediately from runoff, the turbidity gradually clears over a day or two, and then it goes green as the algae bloom takes hold in nutrient-rich water with depleted chlorine. If your pool started brown and has since turned green, you’re now dealing with an algae problem on top of whatever sediment is still in the water.

How fast does a pool turn green after rain?

Algae can double every 24 hours under the right conditions — warm water, sunlight, nutrients and no chlorine. After a summer storm on the Northern Beaches, those conditions are all present at once. It’s entirely possible for a clean, balanced pool to be visibly green within 36–48 hours of a significant rain event.

In winter, the same chemistry applies but the cooler water slows algae growth, giving you more time to act. A pool that might go green in 48 hours in January might take a week in July. This is why Northern Beaches homeowners often get away with less frequent maintenance in winter — but it’s not a reason to ignore a post-rain chemical balance.

The colour of the green matters for how you treat it. Light green or cloudy green is early-stage algae — easier to fix. Dark green or black-green means the bloom is well established. Mustard or yellow-green can indicate yellow algae, which is harder to kill and requires a different treatment approach. A professional water test before you start throwing chlorine at the problem will save you time and money.

What to do when your pool went green or brown after rain

The treatment depends on what you’re looking at. Green means algae — act fast with chlorine. Brown means suspended sediment from runoff — remove the particles first, chemistry second. Getting these the wrong way around wastes time and money.

Brown / Murky

Suspended sediment from runoff

Don’t start with a chlorine shock — there’s no point burning through chlorine on turbid water full of organic matter.

  1. Net out large debris. Leaves, sticks, mulch — remove whatever you can manually first.
  2. Test the water. Check pH, alkalinity and chlorine. Note the values before adding anything.
  3. Add flocculant. Broadcast across the pool surface. Turn the pump off and leave undisturbed for 24–48 hours to let particles settle to the floor.
  4. Vacuum slowly to waste. Set the multiport to “waste” — not back through the filter. Go slowly; disturbing the layer turns the water brown again. Top up water as needed.
  5. Backwash and rinse the filter. The filter will be loaded with fine particles — backwash thoroughly before returning to “filter” mode.
  6. Rebalance chemistry. Adjust pH to 7.2–7.4, alkalinity to 80–120 ppm, restore chlorine. Add a phosphate remover — storm runoff deposits algae fuel.
  7. Shock dose if needed. If chlorine is very low or algae is appearing, shock now that the water is clear enough for it to work.

Don’t run the pump while the floc is settling. Flocculant only works in still water. Running the pump stirs particles back into suspension — turn it off for the full 24–48 hours.

Green (Algae bloom)

Algae bloom from chemical loss

Act quickly — every day you wait, the algae load roughly doubles.

  1. Test the water first. Test pH, free chlorine and alkalinity before adding anything. Adding chlorine to a low-pH pool is mostly wasted — fix the pH first.
  2. Adjust pH to 7.2–7.4. Chlorine is most effective in this range. After rain, pH is usually low — raise with sodium bicarbonate (alkalinity first) or sodium carbonate.
  3. Shock dose the pool. Superchlorinate to 10–20 ppm. A light green pool needs 3–4× the normal dose; dark green can need 8–10×. Use granular calcium hypochlorite.
  4. Brush everything. Brush walls, floor, steps and tile line to break up algae colonies and expose them to the chlorine. Don’t skip this — especially for dark or mustard algae.
  5. Run the filter continuously. Keep it running 24 hours a day until clear. Dead algae turns white or grey and needs to be filtered out. Backwash if pressure climbs.
  6. Vacuum to waste once the algae is dead. Set multiport to “waste” to remove dead algae from the system entirely rather than recirculating it.
  7. Retest and rebalance. Do a full water test once clear — pH, alkalinity, hardness, chlorine and stabiliser. A recovered pool that’s not fully balanced will go green again within days.

Clear in 1–2 days (light bloom) or 3–5 days (heavy). Cooler water slows the process.

Don’t add algaecide while the pool is still green. It’s a preventative, not a cure. Shock with chlorine first, clear the pool, then add algaecide before the next storm season.

How to prevent your pool going green after rain

The best time to prepare for a storm is before it arrives, not after. On the Northern Beaches, storm season runs roughly November through March — the same period when pools are in heavy use and algae growth pressure is highest. Here’s what actually helps:

  • Check the forecast and dose ahead of rain. If there’s a significant storm coming, add a maintenance dose of chlorine the day before. It won’t fully compensate for dilution, but it gives you more headroom before the chemistry falls into algae territory. If your pool is at 2 ppm chlorine going into a storm, it’ll do better than one that was already at 0.8 ppm.
  • Keep your phosphate levels low. A phosphate remover added regularly — particularly at the start of spring — reduces the algae fuel load in the water, making the pool more resilient to a nutrient influx from storm runoff. This is one of the most underused preventatives in residential pool care.
  • Maintain good filtration year-round. A filter that’s running at peak capacity handles a post-storm surge far better than one that’s partially blocked or overdue for a backwash. Regular filter maintenance is the lowest-cost insurance against a green pool — and it starts with the basics: a storm can fill a skimmer basket overnight, so check it straight after any heavy rain.
  • Check your pool within 24 hours of any significant rain. A quick visual check and a 2-minute chemical top-up can stop a light clouding from turning into a full green bloom. The window between “slightly cloudy” and “visibly green” can be less than a day in summer.
  • Consider a pool cover if your garden drains toward the pool. If runoff is your main problem — garden beds sloping into the pool area, or paving that funnels water in — a cover eliminates the direct nutrient influx from storm events. It won’t stop all dilution but it significantly reduces the organic load.

When to call a professional for a green pool after rain

If your pool is dark green, black-green or has visible algae on the walls that’s been there for more than a few days, DIY treatment is usually slower and more expensive than calling someone in. An experienced pool cleaner can assess the water, dose correctly the first time, and run the recovery process efficiently — a two-step shock that turns a three-day DIY job into a faster, cleaner result.

The green pool recovery service covers exactly this situation — storm or otherwise. Craig will test the water on arrival, determine the correct treatment approach for the type and severity of the bloom, and walk you through what happened and how to prevent it next time. If the pool is on a regular maintenance schedule, storm events get handled as part of the visit — no emergency callout needed.

If you’re on the Northern Beaches or Lower North Shore and your pool has gone green after rain, get in touch. Craig usually responds within one business day and can give you a straight assessment of what’s needed and what it’ll cost before any work starts. Get a free quote here.