South-eastern Australia can expect "high intensity" heatwave conditions to elevate fire risks in coming days, with authorities battling to contain out-of-control blazes, particularly in Tasmania.

The island state still had about 50 fires burning on Wednesday afternoon, with 55,000 hectares of wilderness and bushland already destroyed. Forecast gusty winds and a return to high temperatures are expected trigger more fires on Thursday.

Dark clouds loom over Sydney's Botany Bay on Wednesday morning: fire risks are likely to be elevated across south-eastern Australia well into the weekend as another heatwave sweeps in. Credit:Kate Geraghty

There were 70 fires burning in NSW, with 20 of them uncontained, while Victoria has declared a total fire ban for the state on Friday with fire risks set to soar.

Melbourne is forecast to hit 36 degrees on Thursday and 43 on Friday, with the latter dependent on the timing of a cool change. Adelaide is tipped to hit 45 degrees on Thursday, not far off its all-time record of 46.1 degrees set on January 12 1939.