Weather: Coldest night of 2021 for parts of New Zealand ahead of 'dramatic' drop in temperatures

The coldest night of 2021 for parts of New Zealand was just a taster of what's to come later in June, forecasters are warning, with a "polar change" set to bring a dramatic temperature drop next week.

A combination of light winds and clear skies overnight on Wednesday saw some North Island centres record their lowest temperatures of the year, MetService says.

By 7am on Thursday, Rotorua and Hamilton had both dropped below 0C, while Taupo was at 1C, Napier at 2C and Gisborne and Whitianga at 3C. Ashburton, Timaru, Christchurch and Oamaru were all under 3C at the same time.

But Weather Watch says that's just a precursor for a polar southerly that's "about to hack New Zealand's temperatures back".

"It may be warmer than average going into this weekend but the start of next week will be a very different story," said head forecaster Philip Duncan.

"A southerly will move in on Monday and Tuesday from the polar region and will dramatically cut back daytime highs, especially in the south and east of both main islands."

Duncan warns there could be snow on the South Island's alpine passes and highs of around 5-7C for Dunedin and other cities in the lower half of the country.

NIWA meteorologist Ben Noll says the surge of freezing air is likely to arrive from next week, "bringing a chance for snow to low levels and icy, strong southerly winds".

It's a marked change in tack for a month that NIWA Weather says would go down as New Zealand's warmest June on record if it were to end now, thanks to "warm western Pacific seas fuelling sub-tropical air flows" and a "strong polar vortex keeping icy air bottled up near Antarctica".

"Up until June 20, the nationwide average temperature was 10.97C, well above the long-term average of 8.59C," NIWA said in a press release.

"The highest temperature was 22C at Leigh on the 19th and there are now more than 80 locations tracking toward a record or near-record warm June."

Among those locations are Whangarei, Auckland, Gisborne, Whanganui, Palmerston North, Wellington, Nelson, Greymouth, Milford Sound and Wanaka.