How to Lower Pool Water Level After Rain: Valve and Rules

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How do I lower my pool water level after rain? Set the filter valve to WASTE and pump down to halfway up the skimmer. Each 1 mm of rain adds 1 litre per m², so 50 mm on an 8 × 4 m pool is 1,600 litres: about 27 minutes at 1 litre a second. Your water utility decides where it goes.

The short answer

The method: pump on, multiport valve on WASTE, hose to the permitted drain point, stop when the water is halfway up the skimmer box, valve back to FILTER. One valve maker lists WASTE for “lowering pool level / draining pool”.
The litres: BoM says 1 mm of rain gives 1 litre per square metre, so a 32 m² pool gains 800 litres from 25 mm and 3,200 litres from 100 mm, before any run-off from paving.
The minutes depend on the flow you assume: 1,600 litres takes about 53 minutes at Unitywater’s 0.5 L/s cap, 27 minutes at Hunter Water’s 1 L/s cap and about 6 minutes at the 4.2 L/s one pump maker’s turnover table implies.
The utilities disagree on rainwater. Sydney Water says pool water must never enter stormwater; Unitywater will not accept rain overflow into the sewer without approval; Icon Water says rainwater is not permitted to enter the sewer; Hunter Water says no backwash during rain.
Do not overshoot: one pump maker says to keep the water at least halfway up the skimmer box so the pump is never run dry.

How long it takes to lower an 8 m × 4 m pool (32 m² of water) after rain, by rainfall and by the flow rate you assume. Litres from the Bureau of Meteorology’s 1 mm = 1 litre per m²; flow caps from Unitywater (0.5 L/s) and Hunter Water (1 L/s); pump flow implied by one maker’s turnover table (50,000 litres in 3.3 hours at high speed). Rain on the water only; paving that drains into the pool adds more. Our arithmetic, read 18 October 2026
Rain on the pool	Litres to remove	At 0.5 L/s	At 1 L/s	At about 4.2 L/s (pump on high)
25 mm of rain on the water	25 × 32 = 800 litres	about 27 min	about 13 min	about 3 min
50 mm of rain on the water	50 × 32 = 1,600 litres	about 53 min	about 27 min	about 6 min
100 mm of rain on the water	100 × 32 = 3,200 litres	about 1.8 hours	about 53 min	about 13 min
Who sets the flow	BoM: 1 mm of rain = 1 litre per m²	Unitywater cap for maintenance water to sewer	Hunter Water cap for backwash to sewer	Implied by one pump maker's turnover table, high speed

How do I lower my pool water level after rain?

Use the pool’s own pump and the filter’s multiport valve. One valve maker’s instruction sheet lists the WASTE position as “pump waste for vacuuming directly to waste, lowering pool level / draining pool”: the water goes from the pump straight out of the waste port, without passing through the filter.

Check where the water is allowed to go before you open anything (the next section).
Measure the drop. Find the target: halfway up the skimmer opening. One pump maker’s manual says the water should always be at least halfway up the skimmer box so the pump always has water. The drop in millimetres is the gap between the water now and that line.
Turn the valve to WASTE with a backwash hose on the waste port, run to the permitted point, then run the pump.
Watch it. Use the table above to know roughly how long; stay with it rather than walking away.
Stop at the line, turn the valve back to FILTER and restart normal filtration.

If your filter has no multiport valve, the alternatives are a submersible pump or siphoning with a hose, which Sydney Water names as one of two ways to drain a pool.

To lower a pool after rain, set the multiport valve to WASTE and stop when the water is halfway up the skimmer box.

Where can I drain pool water after rain?

Wherever your water utility and council allow, and the five published rules we read on 18 October 2026 do not agree:

Utility	What it publishes	What that means after rain
Sydney Water	“Pool and spa water must not enter the stormwater system, as it’s chlorinated and needs to be treated. Always discharge chlorinated water to the wastewater system.”	No rain exception is published: send it to the sewer, not the street.
Hunter Water (Newcastle, Lake Macquarie)	Backwash to sewer through an overflow gully and air gap, pipe at most 50 mm, at most 1 litre a second; “shall not be carried out during rain periods”; prohibited in vacuum and pressure sewer areas.	Wait until the rain stops, then discharge slowly to the overflow relief gully.
Unitywater (Moreton Bay, Sunshine Coast, Noosa)	Will not accept pool “overflow water”, defined as rain captured in the pool and drained to stop it overflowing, into the sewer unless satisfied there is no safe alternative. Maintenance water may go to the overflow relief gully at up to 0.5 L/s, 10am–5pm and 10pm–5am.	Unitywater points rain overflow to stormwater, a tank or a seepage drain, as your council allows; the sewer needs an application.
Icon Water (Canberra)	Pool overflow from rainwater entry or overfilling: “Rainwater is not permitted to enter the sewer” (guideline dated August 2016).	Not to the sewer; for emptying a domestic pool the guideline expects pumping to the environment or stormwater first.
Water Corporation (WA)	Domestic pool backwash and drainage “are not permitted to be discharged to sewer”; contact your Local Government Authority.	Not to the sewer; your council decides.

So the honest national answer is that the destination flips: Sydney Water treats the water as chlorinated waste for the sewer, while Unitywater and Icon Water treat rain in the pool as rainwater that should stay out of it, because rain is exactly when sewers overflow. Where the sewer is allowed, Hunter Water and Unitywater both cap the flow rate, and Unitywater asks you to watch the gully so nothing floods the property. The full NSW and WA rules are in draining a pool to the sewer in NSW and draining a pool to the sewer in WA.

Sydney Water sends pool water to the sewer; Unitywater and Icon Water keep rainwater out of it. Ask the utility on your bill before you lower the pool.

How long will it take to lower my pool?

Two numbers: litres and flow. The litres are easy. The Bureau of Meteorology’s rule is that 1 mm of rain on 1 m² of horizontal surface gives 1 litre, so litres = pool length × width × rain in mm. An 8 m × 4 m pool is 32 m², so 50 mm of rain adds 1,600 litres, and 100 mm adds 3,200. If paving or a roof drains into the pool, the gain is larger; measure the rise with a ruler rather than trusting the rain gauge.

The flow is the assumption, so we show three. Unitywater caps maintenance water to its sewers at 0.5 litres a second and Hunter Water caps backwash at 1 litre a second. A pool pump on its own runs faster: one maker’s turnover table says its pump filters 50,000 litres in 3.3 hours on high speed, about 4.2 litres a second, 3.5 on medium and 2.0 on low. On WASTE the flow also depends on your hose and plumbing.

Worked: 1,600 litres ÷ 1 L/s = 1,600 seconds, about 27 minutes. At 0.5 L/s it is about 53 minutes; at 4.2 L/s about 6.

The bucket test. To find your own flow, put the waste hose in a 10-litre bucket and time it: 10 seconds to fill is 1 L/s. If your utility caps the rate, the bucket tells you whether the pump on high is over it, and the eco or low speed may be the one that complies.

Litres to remove after rain = pool area in m² × rain in mm; minutes = litres ÷ flow in litres a second ÷ 60.

Can I stop my pool overflowing every time it rains?

Yes, with plumbing rather than a pump. Unitywater’s policy describes the fix: a licensed plumber and drainer installs an overflow pipe from the skimmer box to private stormwater drains that discharge to an overland flow path or the kerb and channel, to a storage tank whose water is used on the garden in dry weather, or to a seepage sump or drain as the local council recommends. Because kerbs and overland flow paths belong to councils, it says to consult the council about the drainage.

Where Unitywater does approve an overflow to sewer, it limits the overflow pipe to an internal diameter of no more than 40 mm, with its invert at least 50 mm above the pool’s top water level. That is the published picture of a passive overflow: it only runs once the water is above the normal level.

If you are building, put the overflow and the backwash destination in the quote rather than solving them after handover; whether pool pipework is plumbing work explains who may connect it, and you can compare pool quotes with that line written in. Builders are listed for Sydney, the Sunshine Coast and Moreton Bay.

A skimmer-box overflow pipe to stormwater, a tank or a seepage drain is Unitywater’s published fix for a pool that overflows in rain.

What not to do when lowering a pool after rain

Do not drain below the skimmer. The pump draws air, and one maker says dry-running damage to the seals is not covered by its warranty.
Do not empty the pool. Lowering 50–100 mm is maintenance; emptying a pool carries a structural risk, explained in why a pool can lift out of the ground. Emptying and refilling a pool is a different job.
Do not run the waste hose into a neighbour’s yard or the street drain unless your utility and council say that is where it goes. Unitywater tells owners to check the gully so water does not reach neighbouring properties or council land.
Do not skip the chemistry. Heavy rain dilutes the water; test it once it is back to level. Why a pool goes cloudy and why a pool goes green cover what rain commonly triggers.

Common questions

How do I lower the water level in my pool?

Turn the multiport valve to WASTE, attach a backwash hose to the waste port and run the pump until the water is halfway up the skimmer box, then return the valve to FILTER. One valve maker’s instruction sheet lists WASTE for lowering pool level.

Can I drain rainwater from my pool into the sewer?

It depends on the utility. Sydney Water says pool water must not enter stormwater and should go to the wastewater system. Unitywater will not accept rain overflow into its sewer without approval, and Icon Water says rainwater is not permitted in the sewer.

How do I lower pool water below the skimmer?

Not with the pool’s own pump: one pump maker says to keep the water at least halfway up the skimmer box so the pump never runs dry. To go lower, use a submersible pump or a siphon.

How do I lower pool water with a sand filter?

If the sand filter has a multiport valve, set it to WASTE, not BACKWASH, so the water bypasses the sand. Backwash reverses the flow through the filter and is for cleaning it.

How much water does rain add to my pool?

1 litre per square metre for every millimetre, on the Bureau of Meteorology’s rule. An 8 m × 4 m pool gains 1,600 litres from 50 mm of rain, more if paving drains into it.

Sources

General information only, current at 2026-10-18. Pool rules differ by state and by council, and penalty amounts are updated regularly. Confirm your own obligations with your council or a registered inspector before acting.

How to lower pool water level after rain