Northlanders, Aucklanders and those along the coast in the Bay of Plenty are being warned to watch out for possible flooding as rain and high winds coincide with a king tide tonight.

Most of the North Island and the top of the South Island was yesterday under a Metservice weather watch for heavy rain or severe gales, or in some cases, both.

Eastern coastal areas between Northland and Bay of Plenty were being warned of the potential for flooding as the king tide coincides with high winds.

Metservice meteorologist April Clark said only one area, Gisborne from Tolaga Bay northwards, and the ranges of Bay of Plenty east of Opotiki, was under a heavy rain warning, but that could change.

Rainfall could total 70 to 100mm in the ranges, with lesser amounts elsewhere, between 3pm today and 6am tomorrow.

But the "biggie" today would be the effect of king tides arriving at the same tide north-easterly winds whipped up heavy north-east swells, Clark said.

High tides of 3.6 metres are due in Auckland at 8.53am and 9.19pm, she said.

"[The swell and king tides] mean we are looking at the potential for sea inundation on the eastern coast from Northland through to Bay of Plenty."

There was still some uncertainty because if winds were not as strong as expected the swell would be smaller, but she urged people living in coastal areas to be aware.

Tāmaki Drive in Auckland was among areas under threat, but anyone potentially affected should keep up to date through local authorities. Auckland Transport warned Maraetai and the Northern Motorway between Esmonde and Onewa roads could be affected.

Vector, Auckland's power provider, said power outages were possible.

A low pressure system arriving in Northland early this morning would bring the wind and rain — with the rest of the North Island affected by mid-morning.

Heavy rain watches are in place for Northland, northern parts of Auckland, Coromandel, western Bay of Plenty, Gisborne to northern Hawke's Bay, north-west Nelson and eastern Marlborough. Strong wind watches are also in place today for Northland, Auckland, Coromandel, Waikato, King Country, Taranaki, Whanganui, Bay of Plenty - west of Kawerau, and Buller.

The foul weather should begin easing tomorrow, Clark said.

"By Tuesday it will be a lot more settled."