Scientific modelling used to predict how bushfires will behave is "not coping" with the current fire situation in south-eastern Australia because the fires are so big, an incident controller says.

Key points: As fires build they create their own weather, so data from the weather bureau becomes less relevant

As fires build they create their own weather, so data from the weather bureau becomes less relevant "Significant movement" of all fires in Gippsland is expected from Friday afternoon through to Sunday morning

"Significant movement" of all fires in Gippsland is expected from Friday afternoon through to Sunday morning For the latest information visit the VicEmergency website.

Andy Gillham, from the Bairnsdale incident control team in Gippsland, Victoria, said in coming days some communities would have fires approaching "from almost all directions".

"Everybody's saying the same thing; and that is that a lot of the scientific modelling that we use to try and predict where fire might run is not coping with what's happening in the landscape just purely because of the fire load," he told ABC Gippsland.

Mr Gillham said as fires build they create their own weather, so data from the weather bureau becomes less relevant.

"We know roughly what's happening but the fires are basically doing what they want in the landscape," he said.

"That's why the state of disaster was declared in Victoria and the state of emergency in New South Wales — because we just need to get people out of the way."

A firefighting helicopter tackles a bushfire near Bairnsdale on New Year's Eve. ( AAP: State Government of Victoria )

Fires exceeded human and computer predictions

NSW Rural Fire Service commissioner Shane Fitzsimmons said the fires in New South Wales on New Year's Eve exceeded all predictions.

"We saw extraordinary fire behaviour with fires, five of them, burning at the emergency warning alert level from 8:00am or earlier," he said at a press conference on Thursday.

"Those fires exceeded all the manual predictions and all the computer-based predictions for what was to be the expected fire spread.

"Those fires spread at what we call the absolute worst case scenario, which typically is not what happens when it plays out on the ground."

He said the RFS had rerun a number of the models and forecasts, as they do routinely.

They are expecting conditions on Saturday to be even worse than on New Year's Eve.

Thousands of people are in evacuation centres across New South Wales, including this one at Bega. ( ABC News: Matt Roberts )

No safe place, only 'safer places'

Mr Gillham said the unpredictable nature of the fires, and the fact that some communities could see fires approach from multiple directions, meant staying to defend properties was only an option for the few extremely well-prepared people who were experienced and understood fire.

But even then, he urged them to think twice.

"We expect significant movement of all fires starting from this afternoon, running through until about Sunday morning when conditions will ease off," he said.

"We just want to reinforce that message. Just leave."

He suggested people go to their nearest evacuation centre or if that wasn't possible, the middle of their nearest large community.

"There's no sugar-coating on this," he said.

"There is no safe place, but there are safer places."