Watercare is warning that unless Aucklanders continue to decrease their water consumption, partially treated water may need to be released into the water supply - and it won't meet our drinking water standards.
The organisation is urging residents to cut their consumption by 20 litres a day until the end of March, following this week's severe weather.
Watercare spokesperson Mark Bourn said that since the announcement of the need for water savings was first made on Thursday, consumption had decreased by 30 million litres - but a further decrease is needed.
"If the savings targets cannot be met, there is the possibility that partially-treated water from the Ardmore treatment plant will need to be put into supply," he says.
"It will no longer meet New Zealand's stringent drinking water standards."
This would mean a boil water notice would need to be issued.
This week's storms restricted the treatment capacity of the city's largest water treatment plant at Ardmore to 50 percent.
That's because the flow coming into the plant contains a lot of silt from slips and erosion in the catchment.
It makes the water look muddy and is more of a challenge to treat. However, Watercare is adamant the water quality hasn't been compromised.
"The quality of water has not been compromised, the issue is the quantity that is available," Mr Bourne says.
"We had two months' rainfall in less than 24 hours."
Production at the four other plants, including Huia and Waikato, are at their maximum levels.
Watercare chief executive Raveen Jaduram says it will take a "sustained period of dry weather" for the water in the dams to settle so the Ardmore Treatment Plant can return to normal activity.
"Our focus is on ensuring our customers receive high-quality water that is safe to drink straight from the tap," he says. "We're achieving this at the moment. But we have had to cut back on the volume of water being treated at our largest plant in order to maintain this quality."
How to conserve water:
- Have shorter showers - cutting each one by two minutes will save around 16 litres
- Turn the tap off while brushing your teeth, saving around four litres
- Don't use your toilet to flush rubbish, saving around seven litres
- Only run your dishwasher and washing machine when they're full.