Some Auckland suburbs have been deluged with an entire average April rainfall in just six hours as Cyclone Debbie slams much if the North Island.

Heavy rain and storms pounded Auckland tonight. The Fire Service Northern Communications Centre says it has taken 120 weather-related calls since 5pm.

Shift manager Daniel Nicholson said calls were mostly from the east of the Auckland, Whangaparaoa and the North Shore.

Flooding at Glendowie in Auckland. Photo / Supplied
Flooding at Glendowie in Auckland. Photo / Supplied

The worst hit has been Kohimarama where a cliff collapsed on to an apartment block.

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Search and rescue were called but no one was injured and residents of the block were tonight hunting for alternative accommodation.

Niwa reported Clevedon had received more than 81mm of rain between 4pm and 10pm Tuesday. The April average for Mangere is 85mm.

Auckland Civil Defence operations manager Aaron Davis said there had been "very localised flooding" across the Auckland region - "like a house flooded, a garage flooded, it's not one region, it's a whole lot of very small ones across the region".

"Looking forward, we are expecting similar downpours for the rest of the evening. We are going to continue to get these small events," he said.

A spokeswoman said the regional civil defence coordination centre was "keeping an eye on things throughout the night".

The damage followed a day of torrential rain across the North Island which forced the evacuation of about 200 houses in Whanganui today and the closure of key highways and schools.

But the worst of the rain is still to come. The Whanganui River level is expected to peak at 1pm Wednesday at Pipiriki and at 7pm in Whanganui City, and the heaviest rain in Auckland is due between noon and 9pm.

Whanganui mayor Hamish McDouall declared a state of emergency at 5.30 last night after being advised the river was expected to peak at 17.7m above normal at Pipiriki - 2.7m higher than a flood in 2015 which inundated part of Whanganui East and the village of Putiki.

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"If the rainfall forecast continues as predicted, it is possible the Whanganui River stopbanks at Kowhai Park will be breached by mid-afternoon," the district council said.

McDouall said residents in "about 200, maybe 250" homes, mainly in Whanganui East and Putiki, should leave their homes by 10am today.

Officials knocked on doors in the affected areas last night to give residents time to prepare.

"That was a problem last time [in 2015], it was dark by the time people knocked on the doors and said people had to be out within a few hours," he said.

"Now we have got the luxury of time. It will never be enough time, but it will allow people to prepare."

The Ministry of Education said all schools in the Whanganui and Rangitikei districts would be closed Wednesday.

"This is automatic whenever a state of civil emergency is declared," it said.

"We will be advising all early childhood education services in those areas to also close."

State Highway 4 is closed by slips between Raetihi and Whanganui, and McDouall said the State Highway 3 bridge at Whanganui might also close this afternoon.

"The bridges are not closed yet, but the reality is that if the flood is at the level it was in 2015, the bridges sustained some damage, so the bridges need to be checked before they are safe to cross," he said.

"So I'd say don't do any unnecessary travel after midday, just to be safe."

He said all residents leaving their homes in the affected areas should register with evacuation centres which have been set up at Whanganui Girls' College and at St Paul's Church hall in Guyton St.

The Waitotara Valley Rd north of Whanganui is also closed by floodwaters, forcing the evacuation of nine students from the tiny Ngamatapouri School, which had to close for three weeks in the 2015 flood.

Elsewhere, MetService has issued heavy rain warnings for the entire North Island and the north and west of the South Island on Wednesday.

Motorists are being warned to avoid the Manawatu Gorge where there have been slips.

Gisborne District Council tweeted at 6.10pm: "Emergency sewer valves opened to prevent sewage overflowing into homes."

In Auckland, the Fire Service tweeted just after 7pm: "Just went north over harbour bridge, there's waves on that motorway."

Fire services responded to flooding in east Auckland, including St Heliers, Mt Wellington, Panmure, Beachlands and Maraetai.

Northern Fire Communications shift manager Daniel Nicholson said Waiheke Island and parts of the North Shore including Takapuna, Devonport and Birkenhead were also affected.

"We've been told tomorrow's going to be worse."