A spring heatwave across large parts of Australia's southeast, including Sydney, will intensify, the weather bureau said, with temperatures expected to peak up to 16C above the September average.

Australia is baking through a rising heatwave which has been building in the country's outback interior over the weekend and is likely last until Wednesday across the states of South Australia, Victoria and New South Wales.

The Bureau of Meteorology said it expected several early spring records were likely to be broken over the next few days, calling the heat "very uncommon for September".

"A reprieve from the heat is not expected until Wednesday onwards, as a stronger cold front crosses the southeastern states," the weather bureau said in a Facebook post.

The heat took its toll on runners in the Sydney Marathon yesterday with 26 people taken to the hospital and about 40 treated for heat exhaustion by emergency services.

Temperatures in Sydney's west are expected to hit 36C before dropping to about 22C on Thursday, the weather bureau forecasts showed.

The heatwave has also elevated the risks of fires with several regions given high fire danger ratings, and authorities urging residents to prepare for bushfires.

About 50 grass or bushfires are burning across New South Wales but all have been brought under control.

Australia is bracing for a hotter southern hemisphere spring and summer this year after the possibility of an El Niño strengthened and the weather forecaster said the weather event could likely develop between September and November.

El Niño can prompt extreme weather events from wildfires to cyclones and droughts in Australia, with authorities already warning of heightened bushfire risks this summer.

A thick smoke haze shrouded Sydney for several days last week as firefighters carried out hazard reduction burns to prepare for the looming bushfire season.