Hazard reduction burning had little to no effect in slowing the most severe fires that devastated more than 5m hectares across New South Wales this summer, an analysis has found.

Forest scientists from the University of Melbourne said initial results suggested hazard reduction was best used in a targeted way around assets to help protect them from less intense fires.

It challenges claims by some politicians that state governments should substantially increase hazard reduction, possibly to meet a target of 5% of land each year. The prime minister, Scott Morrison, has suggested he may introduce national standards that would report on how much hazard reduction the states carried out each year.

Speaking in parliament on Wednesday, Morrison said hazard reduction was at least as important as reducing greenhouse gas emissions to protect people as fire seasons worsened.

The University of Melbourne desktop analysis used Rural Fire Service data to compare the size and severity of this season’s bushfires area with hazard reduction burns over the past five years. The majority of the area in which there had been prescribed burning had been razed again by bushfire in the past three months.

Patrick Baker, a professor of silviculture and forest ecology, said the prescribed burning did not “seem to have done much at all” in areas that faced a crowning bushfire that burned the canopy.

He said the fire that devastated the NSW south coast between Batemans Bay and Jervis Bay over the new year scored 3.95 out of 4 on a severity scale despite recent hazard reduction burns in the area, some of which scored up to 3.8. “Pretty much the whole area was torched,” Baker said.

The analysis found prescribed burning was likely to have helped save property in areas on the fringe of major fire complexes where the flames were less severe, such as Katoomba in the Blue Mountains.

Baker said it suggested prescribed burning was most effective when used as part of a risk-based system designed to help protect chosen assets – human life, biodiversity, property and water catchments – rather than based on an arbitrary numeric target. He said a 5% annual target risked leading to swathes of remote landscape being burned without significantly reducing the risk to assets.

In Victoria, for example, burning the sandy soiled landscape in the Mallee, which makes up nearly a fifth of the state, would do little to protect assets but could have a catastrophic impact on native animals and plants, he said.

“We need to be smart about where we burn,” Baker said. “The other issue is we don’t have endless resources to do this. It’s extraordinarily expensive, so if people want to double or triple or quadruple prescribed burning, the question is what else are we not going to get done?”

Morrison announced on Tuesday that former a defence chief, Mark Binskin, would lead a royal commission into the bushfire crisis, which has claimed 33 lives, destroyed thousands of homes and is estimated to have killed more than 1bn animals.

Scientists and former emergency service chiefs say the climate crisis has exacerbated fire risk and reduced the window each year in which hazard reduction burning can be safely carried out. They say the increased threat demands both greater resources to fight fires and urgent action to cut emissions.

A group of 31 ex-fire and emergency bosses will write to Morrison on Thursday saying the royal commission will be a waste of time and money if it does not examine the role of climate change.

In a statement released on Wednesday night, Greg Mullins, the former NSW fire and rescue commissioner, said: “Climate change has increased fire danger so much that fires now burn freely through areas that have undergone hazard reduction burning recently. I saw fires a metre high burning across mown lawns while fighting fires at Batemans Bay on New Year’s Eve.”

The current Rural Fire Service commissioner, Shane Fitzsimmons, told the ABC on Wednesday that, while some objected to the description, the ongoing fire crisis was clearly unprecedented. He said no fire season had taken as many lives, destroyed as many homes and burned through a quarter of the state’s forested country across the Great Dividing Range. “The reality is it has been an extraordinary, difficult, damaging, destructive, fatal and tragic fire season, and it is without precedent,” he said.

The 5% annual hazard reduction target was recommended by the royal commission into the 2009 Black Saturday bushfires in Victoria, which killed 173 people. Neil Comrie, a former Victoria police chief commissioner who was appointed to oversee implementation of the royal commission’s recommendations, later found the target was likely to not be achievable, affordable or sustainable, and the primary focus should be protection of lives and communities.

The Victorian Andrews government dropped the 5% goal and adopted a risk-based approach in 2016, pledging to focus on reducing fuel loads close to communities and in remote back-country areas where fires can start, grow and build in intensity.

John Thwaites, a former Victorian deputy premier and environment minister and now the chairman of Monash Sustainable Development Institute, said despite claims the states were not doing enough, fuel reduction had risen 61% in Victoria in the decade following the Black Saturday fires compared with the previous 10 years.

Even with that increase, the state met its increased hazard reduction targets only twice before moving to the risk-based system three years ago. It planned to burn 246,000 hectares in 2019, but managed little more than half that because it was not safe to burn, officials said.

In NSW, both the National Parks and Wildlife Service and Rural Fire Service met hectare-based targets for fuel reduction last year, but the rural service fell short of a goal of limiting fire hazards around 113,130 properties.

Thwaites said higher temperatures, drier fuel and stronger winds were increasingly making it unsafe to burn in autumn and spring, but a risk-based approach using sophisticated modelling such as that being adopted in Victoria would reduce the likelihood that homes were lost.