Twelve hours of rain will fall across the top of the country as a deep subtropical low brushes past the North Island overnight, with weather alerts extended , andwarning of 10m-high waves at sea.

Northern regions, including Auckland north of Whangaparāoa Peninsula, are under a heavy rain watch with MetService forecasting periods of heavy rain starting at 8pm and not expected to ease up until until 8am.

In Tairāwhiti Gisborne, where a heavy rain watch has been issued for areas north of Gisborne City for 12 hours from 4am tomorrow, authorities were taking no chances in a part of the country with a long history of being clipped by foul weather pushed down from the tropics.

“Another heads up folk,” Uawa Civil Defence - Tolaga Bay East Coast wrote on its Facebook.

“Everything is already sodden, but be prepared, and keep the safety of your whānau at the forefront of your thinking.”

MetService's 3-day rain radar for 6am tomorrow, showing the deep subtropical low likely to be classed as a 'weather bomb'. Image / Screengrab via MetService 

A heavy rain watch for western Bay of Plenty was also added this morning to those already in place from tonight for Northland, northern Auckland, Great Barrier Island, Coromandel Peninsula and Tasman with the low - likely to be classed as a “weather bomb” - set to brush past the North Island overnight. 

Anyone on a boat near its path might want to keep the seasickness pills close, with Niwa Weather tweeting 8 to 10-metre waves could be expected. 

“Check out the deep low that will pass offshore of the North Island on Saturday! While the worst of this intense system will pass offshore, it is expected to bring some rain to the northern & eastern North Island”, it tweeted. 

“8 to 10 metre waves will be found near its centre!” 

The development of the low was likely to be classed as an explosive cyclogenesis, MetService meteorologist Mmathapelo Makgabutlane said yesterday. 

An explosive cyclogenesis - also known colloquially as a “weather bomb” - is when a low develops very quickly, often associated with intense weather near it. 

But it also looked to be remaining well offshore, protecting the country from worse weather - and would pass quickly with a brief reprieve on Sunday before a new weather system brought showers to the North Island and parts of the South Island, Makgabutlane said. 

“It’s only the rain on its outer edge that looks to brush over the North Island.” 

It's going to be wet in the north and east tomorrow. Photo / Michael Craig 

The heavy rain watch for Bay of Plenty, west of Whakatāne, is already in place and continues until 6am tomorrow, MetService wrote. 

Tasman, west of Motueka, is under a heavy rain watch for 12 hours from 7pm tonight. 

There would be periods of heavy rain, with thunderstorms possible, MetService wrote. 

“Rainfall amounts may approach warning criteria, especially about the ranges.” 

A 12-hour heavy rain watch for Northland, Auckland north of Whangaparāoa Peninsula, and Great Barrier Island begins an hour later at 8pm, while a 9-hour heavy rain watch for Coromandel Peninsula begins at midnight. 

Chatham Islanders have also been alerted to a strong wind watch for 12 hours from 9pm tomorrow night, when easterlies may approach severe gale strength, MetService wrote. 

Cherie Howie is an Auckland-based reporter who joined the Herald in 2011. She has been a journalist for more than 20 years and specialises in general news and features.