‘Going to be quite a shock’: southern Australia set for a heatwave after Christmas

After an unseasonably cold start to the summer, southern parts of Australia are expected to swelter through a heatwave that will peak next week and ease ahead of New Year's Eve.

The weather on Christmas Day itself will be "perfectly warm and sunny" in most mainland capital cities, said the Bureau of Meteorology's Jonathan How. "You really could not have asked for anything better."

The senior meteorologist said the heat will build from Christmas Day into a heatwave stretching from southern Western Australia, South Australia and Victoria to southern New South Wales and Tasmania.

"We are looking at low-intensity to severe-intensity heatwave conditions developing from Sunday and peaking either Tuesday or Wednesday," How said. "So the message to people out and about on Christmas Day and Boxing Day is to stay cool and out of the sun if you can - it's going to be quite a shock considering how cold it was earlier this month."

By Boxing Day, South Australia's temperature will range between the high 30s and low 40s, then likely peak the following day, with Adelaide hitting 41C on Tuesday and inland areas potentially approaching the mid-40s.

South Australia will be relieved by a slight cool change on Wednesday, but it won't reach Victoria until Thursday. That state will hit the mid-30s early next week, reaching 36C on Wednesday.

In NSW, the heat will build in the west on Tuesday, reaching the high 30s. The hottest day along the coast will be Thursday, with Sydney hitting between the high 20s and low 30s.

The heat will dissipate by New Year's Eve and How said people should expect the weather for their celebrations to be on the "cooler side but not a washout".

The bureau is also expecting an elevated bushfire risk in southern parts of NSW, where temperatures are pushing towards the high 30s and mid-40s.

Fire and Rescue NSW has warned that any grass fire outbreaks will likely be larger and more intense than usual due to two years of above-average rainfall increasing fuel loads. It said western NSW has a particularly high risk given the recent flooding.

In northern Australia, a monsoon trough was due to arrive in Darwin on Thursday and lead to widespread rain across the northern part of the country. Large parts of Western Australia, the Northern Territory and outback Queensland are on flood watch.