Waiau recorded the highest temperature in New Zealand since 2011 on Thursday with a high of 37C.

Our scorching summer has moved up yet another a notch with one North Canterbury town hitting 37 degrees Celsius - New Zealand's hottest day in 7 years.

According to Niwa, a weather station in Waiau recorded the temperature on Thursday. The temperature is the highest since February 6, 2011, when Timaru reached 41.3C.

The scorcher came as the country's forecasters warned that the summer's warmest days were yet to come.

"It wouldn't be a stretch to say what's going to happen late this week into early next week, in terms of the spatial size and the intensity, will be the worst of the summer heat ... certainly of what we've seen so far, perhaps for the season as a whole," Niwa principal forecasting scientist Chris Brandolino told RNZ on Thursday.

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"We'll see some really warm to hot temperatures today across the country."

"Air coming from Australia, hot to begin with, gets even warmer as it crosses the [Southern] Alps," Brandolino said.

Brandolino said it appeared the warmest temperatures would be on Tuesday in interior parts of Otago and Canterbury, south of Christchurch.

Temperatures in the upper 30s were a "distinct possibility".

"I wouldn't call it a slam dunk. When it gets to these extreme temperatures, the difference ... could be an hour of cloud cover, or a wind shift," Brandolino said.

MetService meteorologist Tom Bell said it was "a very warm day" across Canterbury on Thursday. It was 29.2C at Christchurch Airport and 34C in Culverden.

Bell said a southerly coming through the east coast of the South Island would bring slightly cooler air from Friday through to Sunday.

"There's still a lot of warm air so it's only just pushing it out of the way."

MetService is forecasting a high of 22C for Christchurch on Friday morning, with drizzle clearing in the morning and fine breaks developing.

RECORDS BROKEN

This summer has been remarkably warm so far, with the January heat following on from a scorching December when, Niwa records show, many places had their warmest mean temperatures on record for the month.

Those places included Nelson where records started in 1862, Levin where measurements date back to 1895, and Ranfurly, which has data from 1897.

Eleven places had their highest December temperatures, including Hanmer Forest with 34.6C, while Waiau had its equal-highest with 34.7C.

The highest temperature recorded in New Zealand is 42.4C at Rangiora on February 7, 1973. The North Island had its highest temperature on the same day – 39.2C at Ruatoria, near Gisborne.

Meanwhile, Wanaka is in the midst of a heatwave – defined as five days when the maximum temperature is 5 degrees Celsius or more above the average maximum for the month – MetService believes. The Central Otago town is forecast to top 30C for the next six days.