The heat is on: Temperatures to shoot up to 35C as heatwave breaks December records

The country will desperately need to cool off on Monday. Photo / iStock

Parts of New Zealand can expect their hottest December day in decades as a mini-heatwave sends temperatures soaring into the mid 30s.

The heatwave will be concentrated in the South Island and the east of the North Island. Tauranga's temperatures will be a warm 26C on Monday.

Metservice is forecasting Blenheim to hit 35C and Christchurch 31C on Monday.

The rest of Otago, Canterbury and the east coast of the North Island can expect to swelter in 30C temperatures on Monday and Tuesday.

MetService duty forecaster Claire Flynn said Tauranga would miss out on the warm winds that were bringing the hot temperatures to areas east of the ranges near Gisborne and Wairarapa, but the city would still benefit from warm temperatures.

"It will be warm, just not exceptionally warm for that time of year."

Ms Flynn said it was poised to be the hottest December day for Blenheim on record.

The previous highest December temperature was 33.2C, recorded in 1948. But it was still shy of the top temperature ever recorded - 37.2C in Gisborne in 1907.

The sweltering conditions are forecast to last for two days.Higher-than-usual temperatures are also forecast for central and eastern areas of the North Island.

Ms Flynn said eastern parts of the South Island would start Christmas week with temperatures at least 8C above average. Gisborne is expected to bask in 29 and 30C at the start of Christmas week and Hastings and Napier are expected to reach a balmy 29C.

In comparison, Auckland is predicted to reach just 23C and Wellington 22C.

But not everybody is celebrating the pre-Christmas heatwave.

Marlborough-Kaikoura rural fire authority fire chief Richard McNamara says a stray spark could be disastrous and emergency teams were poised should fresh fires break out. Fire crews were still monitoring last week's devastating Wairau Valley fire for hot spots after 1200ha of forest was destroyed in a blaze that burned out of control for days.

"We have instigated a total regional fire ban that will come in force at 8am Monday," he said.

A contingent of emergency resources would be on standby including air cover.

"If we do get those conditions developing on Monday we may well put a spotter plane up just to monitor the forestry areas," he said.

He said given tinder dry conditions coupled with steep terrain any potential fire would spread easily and quickly.

People needed to exercise an enormous amount of care in the country's most fire-prone region.

"Mowing in the heat of the day is not a good idea. Having your barbecue with solid fuel is not a good idea. Having a bonfire on the beach next to marram grass is not a good idea. All those things are banned but people may not be fully aware of the total fire ban," he said.

Hot, hot, hot

• Blenheim 35
• Christchurch 31
• Timaru 30
• Gisborne 29
• Hastings 29
• Dunedin 28
• Kaikoura 27
• Whakatane 27
• Nelson 26
• Tauranga 26
• Auckland 23
• Wellington 22