Aucklanders are being told to conserve water after the "more than extraordinary" deluge in the Clevedon Valley this week.

Tap water (file picture). Source: 1 NEWS

If they don't, Aucklanders will be told to boil their water.

Capacity at the city's biggest treatment plant at Ardmore is down 50 percent because of silt from the storm.

Watercare says the clarity of water from the Hunua dams is 100 times worse than usual and more challenging to treat.

It needs people to cut water use by 20 litres, or around two bucketloads, each per day.

The system will cope if savings are made, said Environment and Consents Manager Mark Bourne.

"If not, a different conversation might be necessary."

NIWA says the storm was a one in 100 year event with 225mm falling in 12 hours.