Watercare's dams in the Hunua Ranges are contaminated with silt in the wake of heavy rain.

Aucklanders have fallen 15 million litres short of water savings targets, increasing the risk of the whole city needing to boil water before use.

Watercare chief executive Raveen Jaduram said the targets were easily achievable and suggested Aucklanders brush their teeth with taps turned off and spend two minutes less in the shower.

“If we can’t achieve the targeted water usage savings we may be forced to release partially treated water into the region's supply," Jaduram said.

SIMON MAUDE/FAIRFAX NZ Watercare spokesman Mark Bourne holds silted floodwater from the Hunua Dam.

"In such a situation, a boil water notice would be issued.”

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Aucklanders used 415 million litres of water over the past 24 hours, millions more than Watercare's target usage of below 400 million litres of water per day.

SUPPLIED The silt-laden Hunua Dam.

Water treatment had also been impacted by more storms overnight with about 100mm of rain falling in the Hunua and Waitakere Ranges where nine of Watercare's storage dams are located.

“The clarity of the water entering the Ardmore Water Treatment Plant is still about 100 times worse than what we would expect to see entering the plant.”

Speaking at a press conference on Friday afternoon Watercare's Mark Bourne said the Ardmore Water treatment plant was treating 50 per cent less water than usual in the wake of the storms that battered the region last week.

SUPPLIED The area served by Watercare's Ardmore treatment plant stretches from Waiwera to Pukekohe (area in dark green).

Bourne said the system was under severe pressure as a result of about two months worth of rain falling in less than 12 hours.

Water savings were required because the storm had vastly increased the amount of silt in the water and caused difficulties with water treatment.

David Sinclair, from the Auckland regional health service, said chlorination would not kill cryptosporidium so if partially treated chlorinated water were released into the supply then it would need to be boiled before use.

Watercare's water services as far north as Waiwera and as far south as Pukekohe.

WATERCARE'S FIVE WATER SAVING TIPS

1. Cut your shower time by two minutes (save 16 litres)

2. Turn off the tap when brushing your teeth (save 4 litres)

3. Only run your dishwasher when it’s full.

4. Only run your washing machine when it’s full.

5. Use the half-flush on your toilet.