Firefighters are nervous as a climate scientist predicts temperatures could top 40 degrees Celsius along the east coast today.
Jim Salinger said parts of the South Island's east coast between Timaru and Marlborough could top 40C, while parched Wairarapa and Hawke's Bay look set to swelter in the high 30s.
He said the heatwave was being buoyed by gentle northwesterlies and warm air clashing with the tail end of tropical cyclone Oswald, mirroring conditions that created a 40C-plus day in Canterbury on February 7, 1973, when the temperature in Rangiora hit 42.4C.
National Institute of Water and Atmospheric Research records show the temperature has officially topped 40C on only three occasions, all in the South Island.
MetService forecaster Dan Corbett doubted temperatures would hit the 40s. It was more likely the east coasts of both islands would reach the low and mid-30s, he said.
The hot, settled conditions have Wairarapa principal rural fire officer Phill Wishnowsky on edge, especially after dealing with a spate of fires yesterday, including an unlicensed incinerator fire that threatened a neighbouring property in Te Ore Ore Rd on the outskirts of Masterton.
He urged people to exercise caution and common sense as the temperature soared higher than relative humidity, causing the same dangerous "crossover" conditions that often sparked bushfires in Australia.
"If people want to go around proving their lack of intelligence, then we ask them not to put their hands in their pockets and reach for the matches," Wishnowsky said.
Wairarapa was put on "very high" fire danger alert yesterday, while isolated pockets around Castlepoint, Ngawi, Masterton, Carterton and Greytown had been upgraded to an "extreme" alert.
A total fire ban is in place for Wairarapa and Hawke's Bay, and most of the North Island was placed on high to extreme fire risk yesterday.
Some areas have received less than 5 per cent of their normal January rainfall to date.
NEW ZEALAND'S TOP 40S
- February 7, 1973: Rangiora 42.4C, Christchurch 41.6C, Ashburton 41.3C.
- February 6, 2011: Timaru 41.3C.
- January 22, 1908: Timaru 40C.
The hottest recorded temperature in the North Island was 39.2C at Ruatoria on February 7, 1973.
- The Dominion Post