Adelaide has sweltered through its hottest Christmas on record since 1945, while temperatures in some parts of Victoria hovered around the 40-degrees-Celsius mark.

The South Australian capital reached its forecast high of 40 degrees at 2:29pm on Christmas Day.

Loading...

"It's pretty hot. It's only the fourth time we've reached 40 degrees on Christmas Day since the weather records began," Bureau of Meteorology forecaster Simon Timcke said.

Fortunately, South Australians will have a Boxing Day reprieve with "noticeably cooler" conditions and rain forecast for the capital.

In traditional form Australians took to beaches around the country to beat the heat and shared the fun on social media.

Emergency crews were on standby as heatwave conditions continued to impact on large parts of southern and eastern Australia in the late afternoon.

In Victoria, a maximum of 39.9 degrees Celsius was recorded at Hopetoun, while Melbourne reached 35.5 degrees at about 3:00pm.

Firefighters were battling a grass fire at Woomelang, north of Birchip in the state's north-west.

By 5:30pm, a watch and act warning had been cancelled, and firefighters had the blaze under control.

Patchy rain hit parts of New South Wales, while Brisbane experienced a mostly gloomy day with showers hampering traffic out of the city.

Severe fire danger warnings remain in place for parts of Western Australia, while a major tropical low pressure system is bringing heavy rain and damaging winds to inland regions.

Bureau of Meteorology senior meteorologist Craig Burke says that low will slowly move east.

"We're going to see a fair amount of cloud and moisture build through South Australia over the coming days," he said.

"Heatwave conditions will continue to affect eastern states until Friday."

Turnbull reminds people to help the vulnerable

As Australia sweltered through a steamy Christmas Day, Prime Minister Malcolm Turnbull urged people to remember those who are most vulnerable in the community.

Loading

Mr Turnbull asked Australians to remember those who find Christmas difficult, including the homeless, and thanked those who work over the holidays.

"Of course many Australians continue to work very hard over the break including those who keep us safe, our emergency services personnel, the police, fire and ambulance services, our doctors and nurses keep working while most of us relax," he said.

Mr Turnbull and his wife Lucy helped serve lunch at the Wayside Chapel Christmas Party in Sydney's King Cross, which was attended by about 800 people.

"Please have a very happy Christmas, reach out, give the person next to you a hug, this is a time for love, for community for compassion, but above all for that unconditional love that Jesus taught us — Happy Christmas," he said.

Federal Opposition Leader Bill Shorten used his annual Christmas message to promise he will protect penalty rates.

Speaking with wife Chloe, Mr Shorten also acknowledged the many people who work over the holiday period.

"We think of all those Australians working this holiday season to make our Christmas run smoothly, who get up before the sun rises or work through the night, hard-working people, relying on penalty rates to provide for their families, something I'll always fight for," he said.

Malcolm Turnbull and wife Lucy help serve lunch. ( AAP: Dan Himbrechts )

'Reject tribalism'

In the lead-up to Christmas, Melbourne's Anglian Archbishop had reminded people to reject the growing trend of tribalism which was in stark contrast to the Christmas message of love and generosity.

Archbishop Phillip Freier said those who define themselves by their nation, race, or religion did so while excluding others as outsiders.

This morning, Brisbane's Anglican Archbishop Phillip Aspinall said the Christmas message was more important than ever with so many tragedies.

"It's a message of the possibility of peace and joy and hope," he said.