Key Points:

The Desert Road melted, Christchurch unofficially topped 40C and Kiwis flocked to the rivers and beaches as a heatwave hit New Zealand yesterday.

Weather Watch Centre head analyst Philip Duncan said the Christchurch temperature was a once-in-a-decade recording.

"The hottest period of the year does not usually come until February, but the odds are against Christchurch topping today's high," he said.

Warm air coming across the Tasman Sea has been blamed for the scorching temperatures, which went into the 30s in several North Island centres.

The MetService measured air temperature up to 35.7C in Christchurch yesterday afternoon, close to the January record high of 35.9C in 1979.

Other weather stations in the city recorded unofficial temperatures over 40C, following a similarly hot day on Wednesday.

The Desert Road section of State Highway 1 turned to toffee by 4pm yesterday, its tarseal melting and lifting in the afternoon sun.

Sand was placed on the road surface, and motorists warned to keep their speed down.

"This hot air is part of the weather system that brought temperatures to over 40 degrees in Sydney recently," said MetService weather ambassador Bob McDavitt.

"The air is 20 to 25 degrees coming off the Tasman Sea and warming by around 10 degrees when it crosses the Southern Alps and spreads over the Canterbury Plains."

Christchurch weather analyst Richard Green said some thermometers in the city topped 40C as a hot nor'wester blew in.

"This is like being in the outback of Australia," Mr Green said. "It's been quite some time since I've experienced heat like this."

Temperatures near 30C are forecast for Canterbury again today, but a cooling southerly is expected to arrive on Saturday, which should also affect northern areas.

Mr Duncan said that forecasted temperature readings could be lower than the heat people felt.

"It is important to look at not only pure data - what a thermometer tells us - but also 'polluted data'. This is the effect of hot tarseal, high density housing and metal roofs, which all make us feel hotter.

"Readings are often taken from a white box in a field. But most of us live in the city, so we are more likely to be affected by this polluted data. It may be 26 degrees, but feel like 33 degrees, and a thermometer won't show that."

The dry spell also meant no respite for rain-hungry farmers in Hawkes Bay, North Canterbury and Central Otago.

Federated Farmers adverse events spokesman Frank Brenmuhl said he would be concerned if high daily temperatures and soil moisture deficits in Gisborne-Wairoa and Hawkes Bay continued.

- ADDITIONAL REPORTING: Isaac Davison