Residents of Australia's biggest city, Sydney, sweltered through a sixth straight day of more than 30 degrees Celsius (86 degrees Fahrenheit) heat on Saturday, the longest stretch on record.

The weather bureau said temperatures have been in the mid- to high-30s since last Sunday, the most enduring heatwave since records began 153 years ago in 1858.

"We've had runs of hot weather for three or four days but you get a southerly change that keeps it below 30 then it warms up again," said Bureau of Meteorology senior forecaster Neale Fraser.

Fraser said the heatwave, dubbed the "big sweat", was set to continue into Sunday before cooler weather arrives.

Temperatures hit 41.5 degrees C (107 F) in central Sydney on Saturday, sending thousands to beaches and swimming pools to cool off. Meanwhile firefighters have battled scores of bush blazes.

Others parts of the country are battling extreme weather, after a category five cyclone hit Queensland this week following devastating floods which hit three-quarters of the vast northeastern state.