Australia's fire-fighting resources may be stretched this summer with abnormally warm and dry weather raising the prospects of major bushfires across south-eastern Australia.

All of eastern NSW, almost the whole of Victoria and the most-populated regions of South Australia and Western Australia can expect "above normal fire conditions", according to the Bushfire & Natural Hazards Cooperative Research Centre.

"It is more likely that the resources required to fight bushfires from within a region will be insufficient, with resources required from other areas," the centre said in its November update to its Seasonal Bushfire Outlookk 2014-15.

The revised outlook expanded the fire-risk regions of Victoria, SA and Tasmania, leaving NSW unchanged.

"This summer appears to have greater potential [than usual] for multiple fire events across several jurisdictions," said Stuart Ellis, chief executive of the emergency services peak body, the Australasian Fire and Emergency Service Authorities Council.

As one example of one area that has been drying out, Mr Ellis said soil moisture levels in South Australia were now as dry as they would normally be six weeks later in the year.

The Bureau of Meteorology this week raised its forecast of an El Nino this summer to "at least a 70 per cent chance". Such events typically bring drier and warmer than usual conditions across southern and eastern Australia.

Another warm spell is building across south-eastern parts of the country, lifting fire-danger ratings to severe on Thursday across the Mallee and north parts of Victoria and the Riverina of NSW.

Mr Ellis said a big uncertainty was how long the bushfire season would extend after an early start in many parts of the country.

"That's probably the question that's concerning a number of fire agencies, particularly those which draw heavily on volunteers, because it's potentially going to be a very long period of fire-fighting," Mr Ellis said.

Shane Fitzsimmons, commissioner of NSW Rural Fire Service, said the hot air that had dominated inland areas in recent weeks "had a fairly profound effect on curing the vegetation" in the state's central west and Tableland regions.

The RFS was tackling 20 fires across the state as of Thursday, with seven or eight of them yet to be contained.

Mr Fitzsimmons, though, said the RFS was well placed to wage a drawn-out battle against bushfires, with its volunteer numbers swelling about 6500 over the past 12 months to reach almost 74,000.

"I'm not seeing any evidence yet of a resource drain," he said.