EXPLAINER:

It's a matter of when not if New Zealand will experience the same sweltering conditions described as "absolute hell" in parts of Britain.

A "major incident" has been declared with several large fires raging across London as 34 weather stations around the country recorded their highest ever temperatures.

A record-breaking high of 40.3C was recorded at the Royal Air Force base of Coningsby in Lincolnshire in England's east.

But as it turns out, New Zealand has actually experienced even hotter temperatures than that before.

NIWA meteorologist Ben Noll said the highest temperature on record in New Zealand occurred in 1973 when it surged to 42.4C in Rangiora.

This was when a perfect storm of conditions led to an air mass from Australia crossing the Tasman Sea.

Noll said the characteristics of the air mass almost "returned to their Australian desert-like state" when it descended the Southern Alps into North Canterbury.

"It's more of a matter of when rather than if - I think we will see such temperatures again one day and perhaps as we go later in the century, even more commonly," he said.

The effects of climate change will only increase the frequency of heatwaves and their severity, Noll said.

"You're kind of loading the dice, increasing the odds to observe very very high temperatures."

In the future it wouldn't just be Rangiora with a record-breaking temperature, it could be several locations across the eastern South Island, Noll said.

The type of weather pattern that sent the temperature sky-rocketing in 1973 is the same set of circumstances Noll expected would be associated with similar extremes in the future.

He expected forecasters would be able to issue a warning three to five days ahead of such an event.

While London is literally smouldering right now, New Zealand is pretty warm too.

Analysis undertaken by NIWA meteorologists has revealed the first six months of 2022 were Aotearoa's second warmest on record.

With an average national temperature of 15˚C, the period between January and June was 1.2˚C above the long-term normal.

Noll said if oceans continued to run warmer than average, then New Zealand could very well be in line for intense heat this summer.

But just because it has been warmer than usual and the UK is experiencing a heatwave, doesn't necessarily mean New Zealand will also be slammed by sizzling temperatures.

Firstly, the UK's heatwave is being driven by a surge of warm air being driven off Northern Africa, not Australia.

Secondly, every summer has its own characteristics, Noll said.

Last summer more winds came from the east, off the Pacific Ocean, which meant typical hotspots like Canterbury and Hawke's Bay were not quite as hot if there was a westerly wind, he said.

This far out, it was difficult to predict exactly what summer would bring, Noll said.

"Overall, we're kind of in stride with the rest of the world, which is heating up and seeing more extreme temperatures."