One of the fires, at Tallarook, ignited under severe fire conditions last October and burnt 557 hectares. Another fire at Wensleydale near Anglesea burnt only 11 hectares.

But modelling by the state environment department using the bushfire simulation tool Phoenix Rapidfire, found the Tallarook fire had the potential to burn 7500 hectares.

The modelled bushfire zone included most of the Tallarook State Forest and nearby settlements, including 65 properties.

The bushfire was blamed on a suspected escaped campfire. A report on the bushfire and its interaction with an area of almost 800 hectares treated by fuel reduction burns was produced by the department after the blaze.

It said the fire moved "at a slower pace through the patchy" part of the planned burn, "until the head of the fire was halted where the planned burn coverage was complete". The planned burns were conducted in autumn 2015.

"Field observations and Phoenix modelling confirm that had the fire not been halted by the fuel reduction burn, it is likely to have burnt into adjoining private property under the north-westerly wind," it said.

"The Phoenix modelling, along with the field crew observations, indicate that the planned burning markedly reduced the size and intensity of the fire. This had the effect of mitigating the risk to life, residential properties, other community assets such as livestock and environmental values in the immediate and surrounding areas, as well as providing a safer work environment for ground crews to fight the fire."

The environment department said the Wensleydale bushfire, which ignited in October 2015, had the potential to burn 18,000 hectares including the Alcoa coal mine, and could have posed a serious threat to Anglesea, if fuel reduction burns had not been done in 2011 and 2015.

The department's acting chief fire officer, Darrin McKenzie, said planned burning was "critical to reducing the size and intensity of bushfires".

"The analysis demonstrates that fires such as those at Tallarook-Mt Hickey and Wensleydale last year would have been far more dangerous and destructive without planned burning," he said.

"In both these cases, planned burns conducted beforehand succeeded in reducing the risk to life, homes and environmental values posed by what could have been very destructive fires," he said.

Phoenix Rapidfire was "an absolutely vital component of both our bushfire response and our planned burning activities".

The simulated projections of the potential scale of the Tallarook and Wensleydale bushfires did not take into account the impact of firefighting forces, he said.

The Phoenix Rapidfire tool was the idea of bushfire scientist Kevin Tolhurst, who developed it along with computer software developer Derek Chong.

Associate Professor Tolhurst said it was appropriate to use the tool to assess the potential of the Tallarook bushfire. But he called for such reports to be routinely done after fires to assess what impact planned burning, suppression efforts, resources and vast expenditure actually had on bushfires.

"I guess the main issue I have is that one case is not enough to prove a point, you really need to be taking 1000 fires," he said.

"What they've said about that fire I think is credible, but you can't then generalise and say that's the effectiveness of all burns.

"The outcomes aren't always going to be as good as what happened in the Tallarook instance."

Planned burning, which is fiercely opposed by some people, became a controversial topic last year after a planned burn near Lancefield escaped and became a bushfire, which destroyed four homes, about 26 sheds and was a risk to properties for days.