While New Zealand's Christmas break is caught up in a hemispheric river of moisture stretching down from tropical and sub-tropical regions, fueling heavy rain across the North Island and the top of the South Island, our neighbours in Australia are bracing for an extreme 40s-plus heatwave with temperatures up to 12C above normal.

Australia's national broadcaster reports a sustained heatwave across parts of South Australia, Victoria and New South Wales will see the mercury push above 40 in several locations in the coming days, with 46C forecast for Coober Pedy in South Australia, Mildura in Victoria and Hay in New South Wales.

At the same time, New Zealand's North Island and the top of the South Island is being hit by what the National Institute of Water and Atmospheric Research calls a "fire hose" of moisture stretching down from Papua New Guinea in the tropics.

Niwa called our current wet weather a "striking feature on a hemispheric scale", with the narrow corridor of moisture advecting into New Zealand from the tropics and sub-tropics.

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But with no hope of a cool change and not a drop in sight across the ditch, ABC said this Christmas and New Year period would be the first time Mildura in Victoria's north-west sweltered above 40C for six consecutive December days, with the previous record of five days set way back in 1931.

Adelaide is forecast to hit 41C on Thursday and Sunday, with Melbourne reaching 36C on Thursday, and Canberra 39C on Saturday and Sunday.

Bureau of Meteorology extreme weather meteorologist Sarah Fitton told the broadcaster the heatwave had been building up through central parts of Western Australia.

"No cold fronts to flush that hot air away so it really is just building," Fitton said.

Later in the week, she said, a trough would push the hot air through Western Australia and intensify it across the south-eastern states, with strong winds creating dangerous fire conditions.

Fitton warned Australia's hot flush would be sustained.

"We are looking at a prolonged period of heat," she said, with the broadcaster warning Australians that heat waves were capable of killing far more people than other types of natural disaster. It warned people to dress in lighter clothing to help stay cool, to check on family, friends, and neighbours - and to stay out of the sun.

It also urged people to draw their curtains, blinds and awnings at the start of the day to keep the sun out, to seek airconditioning in places like libraries if they did not have it at home, to look after their pets, and to make sure no children or pets were left in parked vehicles for any length of time.