Temperature records in Canada and the north-west United States are being smashed as a historic heatwave in the region lifts maximums up to 20 degrees above average.

"It's really, really extreme," said Andrew King from Melbourne University's ARC Centre of Excellence for Climate Extremes.

"The records are being surpassed by quite a long way with this heatwave."

Lytton in British Columbia reached a record 49.6C on Tuesday, more than five degrees hotter than Canada's former highest temperature, which, until this week, stood for 84 years.

The record has been broken each day for the past three days, as the heatwave intensified.

Yesterday, climate scientists were astonished when temperatures in Lytton reached 47.9C.

"To beat a national record by 2.5C is, frankly, statistically freaky," Bureau of Meteorology senior climatologist Andrew Watkins said yesterday on Twitter.

By comparison, 47.9C is hotter than any temperature ever recorded in Alice Springs (45.7C) or the notoriously hot US desert city of Las Vegas (47.2C).

Lytton is about 100 kilometres from the Whistler Blackcomb ski resort and is at a latitude of 50 degrees north — closer to the North Pole than the Equator.

Records fall on both sides of border

In the US, Portland, a typically cool, wet city in the Pacific Northwest, reached 46.6C on Monday; that's 5C above a record that until this heatwave stood for more than 50 years.

"It's just breaking records left, right and centre," said Portland-based Australian climate scientist Cassandra Rogers.

Dr Rogers moved from Melbourne to Portland in 2019 to study heatwaves.

This week, she and her colleagues at Washington State University found themselves in the middle of one of the worst heatwaves on record.

"One scientist in particular who's lived here for a number of years, he said he just never thought these conditions would happen in certain parts of Oregon," she said.

"He just never expected temperatures would get that high."

Multiple factors had aligned to produce the unprecedented temperatures, Dr Rogers said.

"There's a really unusually high pressure system that's trapping a lot of heat.

"Heatwaves like this event are also often worse due to the impact of climate change.

"Climate change has increased the frequency of heatwaves; it's increased how hot heatwaves get and increased how large [geographically] they get."

On top of all of that, Dr Rogers said, there was a drought in the Pacific Northwest that had reduced the amount of moisture available to cool the atmosphere through evaporation.

Heat buckles infrastructure

Dr Rogers said despite a huge effort to prepare Portland for the heatwave, unexpected impacts were popping up across the city.

"The main freeway up the West Coast, from California up to Seattle, that's buckled and cracked," she said.

"The train service in Portland closed down today because one of the power cables melted or burst.

"Swimming pools, somewhere where you might want to go to cool down, have restricted how many people go in because they don't have enough lifeguards.

"And there's been a couple of drownings because people have gone to the water to cool down ... the water here is ice pack melt. It's early summer and the water is really, really cold."

Wildlife not adapted to high temperatures

While humans could adapt to some degree to these temperatures through technology, wildlife had not, Dr Rogers said.

"They've had problems with salmon die-off in the past with high temperatures here."

In 2015, extreme heat and low streamflow killed 99 per cent of salmon migrating through the Columbia and Snake rivers in the Pacific Northwest.

In Portland this week, even urban wildlife has struggled.

The heatwave is far from over — the forecast maximum temperature for Lytton on Tuesday was 47C.