Sweltering temperatures will build this week and look likely to break weather records across the country by the end of January.
Photo: 123 RF
The heat steadily increases until it reaches its "apex" at the weekend.
Both islands were experiencing far higher mean temperatures than normal - in some places about 3 to 4 degrees warmer than usual at this time of the year, NIWA Principal Scientist Chris Brandolino told Checkpoint with John Campbell.
Invercargill, for example, was on track for its hottest January on record, he said, with temperatures north of 30°C .
Thursday would be very hot - particularly in the South Island - with low 30s in Alexandra, Queenstown nearing 30° and Christchurch likely to reach 32°.
Some interior places in the east of the South Island could even get as hot as 36°.
In the North Island, Masterton could approach the mid-30s today.
There might be a slight respite on Friday but the "real heat, the apex" would arrive on the weekend, particularly for the South Island.
And night time temperatures were no respite due to the continuing humidity, he said.
"The seas have been unusually warm going back to November - they continue to be unusually warm," he said.
La Nina was a driver for the warm weather, he said, with a lot of north and north-east winds coming from the tropics and sub-tropics.
Mr Brandolino said more humidity and widespread heavy rain could be a possibility for early February.
Officially a heatwave
And the MetService is forecasting a heatwave for Wanaka with a run of highs of at least 30° for the seven days from yesterday.
The technical definition of a heat wave is five days of maximum temperatures at least five degrees warmer than average.
Wanaka's average high for January is 24°, so the forecast will meet the criteria.
Winds over the South Island would turn to the north and combine to create what was called 'foehn heating', creating a fresh burst of hot, dry weather, said MetService meteorologist Tom Adams.
Many other places would be hot, though not technically experiencing a heatwave, said Mr Adams.
Auckland, Northland and Bay of Plenty would be a bit cooler, but much more humid.
Next week Auckland is set to have a run of seven nights in which temperatures do not drop below 20°.
"While they won't be hitting the mid-30s like places down south, it is going to feel just as hot and be unpleasantly sticky as well," said Mr Adams.
The summer has already broken a number of records for highest temperatures in some southern cities, and for the longest run of consecutive dry days.
This month is on track to break the record for the warmest January in New Zealand history, said Mr Adams.
The Climate Change Research Institute said New Zealanders need to brace themselves for change.
Dr Judy Lawrence is Co-Chair of the Climate Change Adaptation Technical Working Group.
She said the country needed to get ready for hotter temperatures.
"As we get hotter in the agriculture sector for instance, we're going to get more pests and diseases, we're going to get more droughts, more fire risks, that's going to have an impact on our ability to respond on our fire services, it's going to affect people who work outside, our seasonal workers."
Dr Lawrence said the changes would also affect forests, the marine environment and even change housing design.
Meanwhile, New Zealand is suffering a fan shortage as Kiwis swelter in record heat and humidity.
Online stores for Bunnings, Briscoes, Mitre 10 Mega and K Mart have all sold out of electric fans.