Heavy rain and possible tornadoes for North Island

The North Island is set to get a drenching from this afternoon, with thunderstorms and possible tornadoes as a low passes over the island.

MetService spokesman Dan Corbett said a large low is moving over the island from the west this afternoon, and will bring heavy rain at times, squally winds, and possibly small hail.

"It's not the afternoon for putting the washing on the line," Mr Corbett said.

"There's a good line of persistent rain with embedded thunderstorms in it sitting to the west of Northland and Auckland, through the afternoon that is going to be barrelling its way through. So it is going to be a wet thundery commute home for people."

Mr Corbett said there is a high risk of thunderstorms from Northland down to Taranaki, across to the Bay of Plenty.

"It's quite likely we will see some lightning in Auckland city and with those there will be some heavier downpours, maybe the risk of some small hail and there is always just that outside risk, if the winds are in the right direction, of a possible small funnel or tornado."

The rain will continue overnight, coupled with thunderstorms and squally winds, Mr Corbett said, as the low continues to move over the North Island.

"There is still thunder tomorrow morning but [it is] turning more showery and the showers get smaller and smaller so they are not as intense as we go through Thursday. And then Friday is actually a dry day. Or at last drier."

Saturday will start off fine for the top half of the country, Mr Corbett said, however another front that is over the South Island will reach the North Island later in the day.

In the South Island rain levels may reach warning levels in Fiordland and Westland late Friday and Saturday, as the front moves over the island. The rain may also spill over to the Otago and Canterbury headwaters, reaching warning levels.

Northwesterly winds are forecast to become very strong and gusty east of the Southern Alps, and also about central New Zealand. These gust may become severe gales on Saturday from central Canterbury to Wairarapa.