Multiple heavy rain watches are in place all across the country for the end of the week, with upgrades to warnings possible, MetService warns.

The culprit is a front preceded by a moist northerly flow expected to move slowly east across New Zealand on Thursday and Friday.

"It takes it time to run in from the west on Thursday," says MetService meteorologist John Law.

Fiordland, north of Doubtful Sound and the ranges of the Westland District was forecast to be hit first, with period of heavy rain from 11am on Thursday.

Northland will start to see the wet weather hitting the area in the afternoon from 1pm on Thursday.

Meanwhile, the heavy rain will sweep across Auckland, including Great Barrier Island and the Coromandel, Bay of Plenty and Rotorua later in the evening on Thursday with possible thunderstorms.

Taranaki and Tasman, northwest of Motueka also have heavy rain watches in place from Thursday evening onwards.

Meanwhile, it has been a cold start to Wednesday with Wanaka, Ashburton, Queenstown, Christchurch and Timaru all waking up to below freezing temperatures.

MetService says the coldest spots are in towards Tekapo where the temperature dropped to -7°C on Wednesday morning.

-RNZ.