Heat grips parts of Australia, fire danger escalates

Millions of Australians are enduring blistering temperatures, with parts of New South Wales tipped to pass 40 degrees.

The heat is spreading across the country with Sydney, Canberra and Perth bracing for temperatures in the high 30s.

Western Sydney's suburbs could swelter in temperatures as high as 40 or 41 degrees, while Newcastle and the Central Coast will hit 38 degrees.

The Central West region will bear the brunt, with Dubbo forecast to reach 42 degrees and Forbes an oppressive 43 degrees.

Extreme fire danger has been declared in parts of Victoria and South Australia, prompting total fire bans across high-risk regions.

Authorities warned that bushfires could quickly become "uncontrollable".

"If any fires do get going in these extreme areas today, they're likely to be uncontainable," said Bureau of Meteorology senior meteorologist Dean Narramore.

"So dangerous fire conditions with a dry lightning threat as well."

While the heatwave continues to grip much of the country, a severe weather warning has been issued for New South Wales as conditions were expected to shift to thunderstorms later today.

"Severe thunderstorms could bring large hail, damaging winds, and heavy rainfall across northern and eastern NSW," Narramore said.

However, the Bureau of Meteorology predicted the storms would bring a much-needed drop in temperatures across the country.

Meanwhile, a marine heatwave off Western Australia's Pilbara coast was believed to have killed around 30,000 fish at Gnoorea Beach.

Authorities have described the incident as climate-driven, with ocean temperatures reported to be four to five degrees above the long-term average.

The Bureau of Meteorology expected these conditions to persist through February.

This marine heatwave has been likened to the devastating 2010 event, which destroyed 1300 square kilometres of seagrass in Shark Bay.

The effects of that incident were still impacting commercial fisheries today.

Fisheries officials continued to monitor the situation as the environmental toll of this latest heatwave unfolds.