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A heatwave forecast to sweep across Australia in coming days could escalate conditions for the nation's bushfires, authorities fear.

Temperatures are set to hit over 40C (104F) from Friday in several bushfire-affected states including New South Wales, South Australia and Victoria.

There are more than 100 fires burning, with the largest to the west of Sydney.

Firefighters took advantage of cooler temperatures over Christmas to try and and contain fire fronts.

"[It] is all about shoring up protection before we see the conditions deteriorate again," said New South Wales Rural Fire Service Commissioner Shane Fitzsimmons on Friday.

The surge of heat is due to return just a week after Australia recorded its two hottest days on record, consecutively. On 19 December, the national average maximum hit an all-time high of 41.9C (107.4F).

Fire officials said they were bracing for similar dryness, low humidity and volatile winds. Last week, dozens of homes were lost in rural towns south-west of Sydney from a flare-up in the blazes.

"We're not expecting the catastrophic conditions like we've seen in the last few weeks... but it's certainly going to be another tough period," said Mr Fitzsimmons.

Holidaymakers have been warned to check the status of major highways before setting out, after some roads were cut off last week due to bushfires.

Massive blazes to the south-west of Sydney could also pose a threat to the city's drinking water supplies, reported the Sydney Morning Herald.

Calls for firefighter payments

Australia has been ravaged by bushfires which have killed nine people and razed hundreds of homes since September.

As the fires rage on, Australia's government has been criticised for its response to the crisis and its reluctance to acknowledge the impact of climate change.

Public debate in the past week has centred on compensation for volunteer firefighters, who make up almost 90% of the responders battling the blazes in New South Wales. Last week, two volunteer firefighters died while en route to a blaze near Sydney.

Prime Minister Scott Morrison has consistently rebuffed suggestions of payments to volunteers. Instead, on Tuesday, he announced an extended paid leave entitlement for public servants who went to fight the fires.

However government minister Darren Chester broke ranks on Thursday, suggesting payments or a tax was necessary in the long term.

"Once you move to campaign fires - fires that go weeks and months - we have volunteers taking a long time away from work," he told the Australian Broadcasting Corporation.

"How much pressure can we keep putting on them?"