Residents in the Bay of Islands and Whangārei are being warned to urgently reduce water use after floodwaters knocked their water treatment plants out of action.

Torrential rain in recent days, along with large amounts of silt and debris washed down the Waitangi River, have damaged a Far North District Council treatment plant just upstream of Haruru Falls.

The plant supplies water to Paihia, Ōpua, Waitangi and Haruru Falls.

Both the water intake pipe and a submersible pump are thought to have been damaged.

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Whangārei is grappling with a similar emergency after two of the city's three water treatment plants were affected by the deluge.

Whangārei residents are being urged to use water for cooking, drinking and essential hygiene only until repairs can be carried out.

In Paihia the council is in the meantime supplying treated water from its storage reservoirs.

However, due to lingering effects of the drought and a large number of visitors in Paihia during the school holidays, those reservoirs are only about 25 per cent full.

Far North District Council infrastructure manager Andy Finch said it was vital that water consumption was reduced immediately to prevent the reservoirs from running dry.

"We're asking all households and businesses to defer non-essential water use. That means not washing clothes for at least 24 hours, taking shorter showers, and flushing less. Many people will also be keen to wash away debris from yesterday's deluge, but I'm asking them to defer that work for now."

Finch said contractors were working around the clock to get the treatment plant running again and the council was using water tankers to replenish the reservoirs with treated water from elsewhere in the district.

"That is helping, but the real solution is reducing water consumption until we can again get normal river flows through the treatment plant. At this stage we don't know how long that will take," Finch said.

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In Whangārei, which was hit by what meteorologists have described as a 1-in-500 year downpour, pumps at the Poroti water treatment plant were flooded and need to be dried, a process which takes at least 24 hours.

Whangārei District Council water services manager Andrew Venmore said it was unlikely the plant would be running again before Monday.

The Whau Valley treatment plant was also offline as staff tried to improve water quality.

It was not clear how long that would take with the best-case scenario late on Saturday evening but it could take until Sunday.

''It's important everyone in the Whangārei water supply area saves water now as all reservoirs are dropping and only the Ruddels treatment plant is operational,'' Venmore said.

He appealed to Whangārei residents to skip having a shower or a bath tonight, leave the dishes, and don't even flush the toilet — unless really necessary.

According to the Northland Regional Council the area worst hit by Friday's deluge was initially the Waitangi River catchment with the thunderstorms moving south to Whangarei in the evening.

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