When the vehicle's wheel caught on the log, the 250 kilo quad rolled over him.   

Mr  Brown's ribs were broken and shoulder injured, and he needed a hip replacement. The accident reduced his ability to work by 70 per cent for three years.

"Had I been pinned under it, I may well have suffocated,'' Mr Brown said.

Quads are advertised as an "all terrain vehicle", but even a "tuft of grass" is enough to destabilise one, according to research published in Safety Science.

The research adds to the growing evidence that has found quad bikes - a booming $230 million market - are unsafe without rollover protection.

Since 2001, there are have been more than 200 quad-bike related deaths in Australia. About 64 per cent of those occurred on farms, which makes quad bikes the biggest killer of workers in Australia, said NSW Minister for Better Regulation Matt Kean last year when he launched a safety program to encourage people to choose the safest vehicle for the terrain.  About 8000 people have been hospitalised with serious injuries caused by quads.

The latest research found quads were "particularly vulnerable to a rollover, even when travelling around farming environments where they traverse relatively small bumps on typical grassy slopes at moderate speeds".

"Such small bumps are not readily perceivable as a hazard. Moreover expecting a surface on a grassy hill to be smooth is not realistic within a farming environment," said the paper by David Hicks and Professor Raphael Grzebieta of the University of NSW's Transport and Road Safety Research.

Based on the finding, they say  quad bikes "should no longer be considered a vehicle fit for farming tasks and alternative safer vehicles should be considered".

For the research, Mr Hicks modelled a quad bike going over a bump at a range of speeds and on a range of different slopes, including flat ground.

As the angle of the slope increased, the size of the bump needed to roll the vehicle became smaller.

An earlier report found almost one-third of quad bike fatalities occurred on terrain with an incline, and almost half occurred on uneven ground with fallen logs.

The chairman of the non-profit group Farmsafe Australia Charles Armstrong has been lobbying for a five-star safety ratings system to motivate manufacturers to improve new machines.

Because it would only cover new vehicles, he also wants existing quads to be fitted with crash or crush protection devices, such as rollover bars. In Australia, there hasn't been been a serious injury or fatality on a quad with crash protection, Mr Armstrong said.

Quad manufacturers "won't accept responsibility for what they have done" because of fears of liability, he said. Although government agencies recommend crash protection, resellers often discouraged buyers from buying rollover protection, Mr Armstrong said.

Quad bikes were practical and manoeuvrable machines, but they had to be respected "because they're probably the most dangerous bits of machinery ... on the farm", said Mr Armstrong.

Legislation may be required to mandate the use of rollover protection if the manufacturers continue to thwart efforts to improve the vehicles safety, he said.

The NSW Farmers Association, including Mr Brown who is an executive councillor, reject the need for additional legislation or regulation making crash protection compulsory. Its special counsel Matt Waring told Fairfax Media that the group endorsed incentives such as a ratings system, better education and training and greater use of existing rebates on helmets and the installation of crash protection devices.

Mr Brown used to train workers before he let them ride a quad, and even then there were a few minor incidents. Now he won't let anyone else other than him on the quad, saying he knows his land and his machine best - and he drives cautiously and wears a helmet.

"I still see quads as a useful tool ... it's about them being fit for purpose," said Mr  Brown, but he recommends others should purchase a side by side vehicle instead if they can afford the more expensive vehicle, which has seat belts, and rollover protection.

The Federal Chamber of Automotive Industries, which represents quad bike manufacturers, recommends "active riding" where a rider stands or leans to shift the centre of gravity on a slope or a bump. Many farmers jump off their moving quad - if they think a rollover is likely - to avoid being caught underneath.

The chamber also rejects claims that crash protection would improve safety.

But Dr Emma Webster from the Sydney University's School of Rural Health in Dubbo said the industry pushed the view that accidents only happen when the rider is "doing something stupid" such as going too fast.

Farmers she's interviewed say they're very careful, and, like Mr Brown, "extremely particular about who was allowed to ride a quad on their property".

She said Mr Hicks' research confirmed that quads rolled not when a farmer was doing something stupid, but when they were doing something the manufacturers say they should be doing.

"They are producing a commodity which is not fit for purpose," she said.

smh.com.au