With the mercury hitting 31.8C on Monday, Whangarei had its warmest day since records began half a century ago.

A combination of hot air from across the Tasman, northwest winds and dry ground conditions in Northland helped raise the temperature which broke the previous February record of 31.4C set Tuesday last week.

The National Institute for Water and Atmospheric Research (Niwa) began records for Whangarei in 1967.

Niwa meteorologist Ben Noll said 31.8C at 4.30pm was a "very hot day" by Whangarei standards. It was not, however, a Northland record.

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Kerikeri's warmest temperature on record was 35C on February 2, 2009, Dargaville hit 32.4C on February 20, 1998, and Kaitaia's hit 30.6C on February 18, 1974.

Mr Noll said there were three reasons behind Monday's record temperature.

"Look to Australia. A very impressive heatwave occurred over the weekend and into Monday as many locations set all-time record warm temperatures there."

Queensland and New South Wales are battling temperatures in the early to mid 40C, with people and animals treated for heat-related illnesses.

Queensland recorded 47.2C - the state's hottest ever February.

Mr Noll said some of the heat swept across the Tasman Sea on Monday, and headed for Northland.

"As the air passed over the higher terrain in western Northland (Tutamoe Ranges), it is forced to descend as it moves toward Whangarei.

"As the air descended the ranges, it warmed and reobtained some of the hot qualities it had when it was over Australia. This is known as a downsloping wind or a foehn wind."

Mr Noll said dry conditions in Northland meant ground heat radiated all over the region and created hot sticky conditions.

No record temperatures were expected for Whangarei later this week with rain up to 75mm forecast for some parts of Northland from today, continuing through to Friday.