Weather: Sub-tropical storm bringing gale-force winds, flood risk to North Island

Most of the North Island is at risk of flooding as a severe storm approaches New Zealand.

Heavy rain and gale-force winds are gaining momentum the deep low heads towards the country.

NIWA weather warns there are similarities between this sub-tropical storm and Cyclone Ita, which closed Tamaki Drive with heavy flooding.

MetService meteorologist April Clark told Newshub king tides - "basically a normal tide that's been amplified" - combined with unusually high tides, means waves could be coming up onto coastal or low-lying areas from the Bay of Plenty up to Northland.

The storm is set to move over the North Island on Thursday, dragging with it a lot of moist, sub-tropical air and strong nor-easterly's.

MetService expects it will need to issue a heavy rainfall warning for Northland and Auckland on Thursday, and in Coromandel Peninsula and Bay of Plenty in the evening.

The rain will be heaviest in the upper north - but "the lower half of North Island will be getting it a bit more into Friday," Ms Clark told Newshub.

"But as time goes on the position of that low becomes more uncertain... [it's] very compact, exactly where that low centre is could change where rain band will be."

Winds will be strong in exposed areas, and could bring trees down or leave debris on the roads, Ms Clark says. A severe gale warning is expected for the northern half of the North Island during Thursday and Friday.

"The aftermath could be more from the wind."

Ms Clark warns the South Island isn't in the clear either.

"The South Island could get a few thunderstorms as well, but not as widespread as North Island - it could be isolated spots."

"This is the most significant low since mid-September - we've been getting thunderstorms but normally isolated, this is broad.

"Everyone will get a dose of rain."