One person has died and dozens have sought medical treatment during a scorching heatwave that has baked most of Australia.

A woman collapsed and died while walking near Cape Otway in Victoria's southwest on Friday when temperatures peaked above 40C.

The coastal fringes of southern states received a cool change on Saturday, but the Bureau of Meteorology said it would be a temporary respite.

"Tasmania, Victoria and South Australia have experienced a cool change but the rest of the continent is hot," the bureau's Alasdair Hainsworth told AAP.

"That tremendously hot air is still hovering just to the north."

Temperatures in Hobart dropped to 25C on Saturday after hitting 41.8C on Friday, the city's hottest day since records began.

Parts of Tasmania's east coast were cut off by bushfires and a blaze on the Tasman Peninsula was out of control on Saturday evening.

Conditions across Victoria also cooled, after the temperature soared above 40C across most of the state on Friday afternoon.

The state's hottest towns were in the northwest, with Walpeup and Hopetoun Airport reaching 44.8C.

Dozens of people sought treatment for heat-related illnesses and the state's CFA website crashed for several hours as concerned Victorians sought bushfire updates.

A number of total fire bans have been implemented across SA, with 50 fire crews watching for flare-ups from a controlled bushfire on the Fleurieu Peninsula.

The threat of more fires eased after cool change swept through much of the state, dropping temperatures in many parts to the high 20s.

Some parts of SA reached temperatures in the high 40s on Friday, with Wudinna, on the Eyre Peninsula, hitting 48.2C.

The NSW Rural Fire Service has established total fire bans in the northern and southern Riverina districts and the lower and upper central west plains.

Firefighters fought a blaze on the mid-north coast in the Booti Booti National Park near Forster that caused the closure of roads in the area and isolated the town of Green Point.

The small western town of Hay baked as temperatures soared to 48C.

Canberra and Perth had the highest capital city temperatures on Saturday, with both reaching 37C and the same predicted for Sunday.

The bureau expects temperatures across the country to rise on Sunday and Monday before cooling midweek and rising into the 30s again next weekend.

Prime Minister Julia Gillard reminded people extreme heat was widespread.

"The best thing people can do is stay in touch with local authorities" and heed local community warnings, she said.

Health bodies are urging people to drink plenty of water and stay out of the heat.

The National Heart Foundation is advising those with heart disease to take it easy during the next few days, with studies showing an increase in heart attacks and death from extreme heat.

Police have warned that leaving children, elderly people or pets in cars could prove fatal in hot weather.