Amount of rain slamming North Island 'unusual' - NIWA

A NIWA forecaster is warning that the heavy rain slamming parts of the North Island this week is "unusual", with some places getting four times more rain than they normally would for the whole of January - and there could be more on the way next week.

Forecasting scientist Chris Brandolino said an atmospheric river is behind the bad weather, explaining the weather feature is bringing moisture from the tropics and subtropics to New Zealand.

"Imagine a river flowing through rural NZ...this is a river of moisture coming from the tropics, but it's in the atmosphere," he said.

"And that brings down with it some high octane fuel and a lot of water vapour, which is fuel for rainfall.

"It's moving over the Coromandel Peninsula, it's very mountainous terrain - so basically imagine your hand squeezing a sponge, those mountains do a really good job of ringing out that sponge and that's what we are seeing."

Some places are getting three or even four times the amount of rain they normally would for the entire month of January in just a few days, Brandolino said: "That's where you can get these issues like flooding and slips.

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"It is unusual, definitely, when you're talking about a lot of rain in a short period of time."

There is flooding and road closures in Coromandel today.

He urged people to keep up to date with the latest conditions, particularly campers in Coromandel and north-west Bay of Plenty, warning that the weather could be "very wet to put it mildly" until Saturday afternoon - and there could be a second wave to come.

"Somewhere near the early to middle part of next week, there could be another weather system coming down from almost the same area in the tropics and subtropics and that could bring another round of heavy rain for parts of the North Island.

"And if that occurs over the same area, which is currently getting the heavy rain, that could make things bad," he stressed.

"The Coromandel, the Bay of Plenty, this area is really gonna be under the pump for continuous rainfall and where the rain falls heaviest, those rainfall amounts are going to be really impressive."