Tropically-charged rain and gales are bearing down on the North Island, with an atmospheric river set to unpack a torrid week of wet weather on Auckland and surrounding regions.

One forecaster says at least one month’s worth of rain - possibly more - is forecast to fall until Friday with up to 200mm predicted for parts of the top half of the North Island and the South Island’s West Coast from Sunday to Friday.

The rain is forecast to start falling on Saturday evening across Northland before the system moves into Auckland, Coromandel and Bay of Plenty on Sunday.

Auckland will see eight days of wet weather from Sunday, the first few days with heavy downpours and gales forecast.

The first tranche of weather alerts have been issued by MetService this morning.

A Heavy Rain Watch is now in place lasting 42 hours for Northland from 9pm Saturday, with Auckland’s rain is set to start before dawn on Sunday.

A Strong Wind Watch has also been issued for Auckland and Northland across the weekend through to Monday afternoon.

The wind is expected to whip up wild seas with waves up to 6m crashing against the upper North Island eastern coastline on Monday night.

MetService forecaster Jessie Owen said the sub-tropical low-pressure system packed with strong winds and rain was bearing down on New Zealand dragging a lot of warm wet air with it.

“It’s going to bring rain to the most northern parts of the North Island such as Northland late on Saturday and then spread south throughout the weekend.”

Owen said some of the rain would be heavy. It would also be partnered with strong northeasterly winds that would whip up wild seas.

Weatherwatch.co.nz this morning warned the gales had potential to be damaging, gusting up to 90 km/h in exposed parts.

“This is enough to cause isolated power cuts and trees/branches down,” said forecaster Philip Duncan.

She said next week the low would keep tracking south dragging the rain across most of the country, unleashing the worst weather on northern and western regions.

“After that low moves to the south we’re into a general wet disturbed northerly flow.

“We’re looking at several bands of rain coming in from the north. It’s all looking a bit messy and not particularly well defined but wet.”

Auckland, Northland, Coromandel and the Bay of Plenty were expected to fare the worst and be placed under heavy rain alerts.

“We don’t have any warnings out just yet but we probably will within the next day or so,” she said.

The same regions would also be at risk of strong winds and big swells.

Owen said after a few days of chilly nights people would see a return to sticky, humid conditions.

Auckland day-time temperatures are forecast to remain above 20C for the coming week as the tropically-charged wet spell swamps the atmosphere.

“Once this warmer air comes down we’re looking at some pretty warm overnight temperatures in the mid-to-high teens.

“There’s not going to be much cooling between day and night from Saturday night in the Far North.”

Niwa said the atmospheric river would import water vapour from the tropics - or fuel for rain - increasing the risk of heavy downpours.

It would also mean unusually warm temperatures.

“Since the low originates from the subtropics, it will drag warm, moist subtropical air over New Zealand, raising overnight temperatures into the teens for most of the North Island.”

Weatherwatch said the high levels of moisture and warmth from the tropics could produce thunderstorms. The main risk would be from Wednesday to Friday next week.