Canterbury is "staring down the barrel" of 30 deg C temperatures this weekend, while others face the prospect of "extreme weather", surface flooding and slips.
Eastern parts of the South Island, including the Canterbury Plains and Marlborough, are due to hit 30 deg C, while Christchurch city is set to reach at least 28 deg C on Saturday.
Sunday would see temperatures cooling slightly, MetService meteorologist Alex Holden said.
Parts of Canterbury would see the most significant drop - from 30 deg C to about 23 deg C.
In Otago, Alexandra looks to be the warmest spot over the weekend, with 27 deg C on Saturday and 24 deg C on Sunday, while Dunedin is looking at highs of 22 deg C and 19 deg C.
In the North Island, warm conditions would continue on Sunday, with highs of about 28 deg C for Hawke's Bay and Gisborne.
The South Island's West Coast could remain "quite showery" on Sunday with the front stalling nearby, but no significant rainfall was expected, Holden said.
Showers "here and there" would be seen in other parts of the South Island but nothing particularly significant.
New Zealand would be in a bit of a "holding pattern" over the next few days, Holden said, with the North Island largely settled and on and off rain for the West Coast of the South Island.
For parts of the North Island, the mugginess isn't likely to subside until at least Sunday - if at all.
Masterton could be the place to be, with warm weather, no rain and not much cloud expected, either.
But MetService is warning of extreme weather on the West Coast of the South Island; a heavy rain watch has already been issued for the ranges of Westland District, from 1am until 9pm on Saturday.
Periods of heavy rain may exceed warning criteria, especially around the glaciers, and there is a high change the current watch will be upgraded to warning level, according to MetService.
"Streams and rivers may rise rapidly. Surface flooding, slips and difficult driving conditions are possible."
The will be "bits and pieces" of showers for other parts of the South Island, Holden said.
Meanwhile, the North Island is looking dry for the most part.
However, it would be "muggier and warmer than people will like".
