Five farmers killed in quad bike accidents all had trailers or spray units fitted to their bikes, and two had been using cannabis, a coroner has found.

Coroner Brandt Shortland has released his findings in the five deaths and a two-day inquiry into quad bike safety held in April this year.

The common factor in the deaths was use of an after-market spray unit or trailer attached to the bike, and Coroner Shortland said there was a lack of research on how these affected stability.

Two of the farmers had also been using cannabis, which the coroner said could not be ruled out as a factor.

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The coroner said quad bikes were "a farmer's best friend and their worst enemy".

Safety was seriously compromised when bikes were used beyond their capability, and they were prone to rolling and tipping in a range of circumstances.

There was a "Mexican stand-off" between competing factions over the need for roll-over or crush protection devices, with manufacturers arguing they made bikes unstable, but academics and engineers saying they improved safety.

In Australia, roll-over protection devices (ROPs) have been fitted to quad bikes used by government employees.

Coroner Shortland said in the next five to 10 years this would provide clear evidence on whether the devices prevented injury.

There was strong argument that ROPs should be considered, and while not perfect, they would more likely save the life of a rider than not, he said.

He endorsed the FarmSafe Quad Bike Licence programme, where farm workers were trained on bike safety and maintenance, but said a more widespread licensing and registration regime would be difficult to establish.

There had been a marked increase in use of helmets, which was one of the key messages of an Ministry of Business, Innovation and Employment quad bike harm reduction project started in 2011.

The coroner endorsed the campaign's four messages, which also including proper training for riders, using the right bike for the job, and preventing children from riding adult bikes.

Farmers should also consider carrying a personal alarm, as many were trapped under their bikes for some time before succumbing to their injuries.

The five quad bike deaths:

*Carlos Frederick Mendoza, 52, died on his Dargaville farm between 14 and 16 September 2010, of positional asphyxia by having his body forced into an awkward position by the bike. Mr Mendoza wasn't wearing a helmet, had a rear-mounted spray unit on the back of the bike, and had traces of cannabis in his system at levels suggesting he may have been affected by the drug. Poor bike maintenance, including low tyre pressure, and the effect of the fluid in the spray unit, contributed to the roll-over.

*John Roderick McInnes, 64, died on his Hikurangi farm on September 25, 2010, of mechanical asphyxia. He had a spray unit mounted on the bike, and had been spraying a steep paddock when he was found face-down, with the bike on top of him. He was thought to have gotten off the bike to try to steady it, and lost his footing and been crushed. The steep, uneven surface contributed to the instability of the bike.

*Suzanne Claudia Ferguson, 62, died on her Kaikohe farm on August 9, 2010, of mechanical asphyxia. She had been feeding out silage from a trailer attached to a quad bike, and was found pinned by the bike against an electric fence. The bike appeared to have lost traction while going up a steep hill.

*Willem Lambertus van der Pasch, 57, died on his Otorohanga farm on September 2, 2011, of a head injury. He had been towing a trailer loaded with hay, and was found trapped under the upturned bike. He was not wearing a helmet, which could have reduced the head injury. The coroner found a moment of inattention may have led to him suddenly over-correcting and tipping the bike. The load on the trailer may have contributed.

*Grant Charles Cornelius, 53, died on his Dairy Flat farm on September 5, 2011, of positional/traumatic asphyxia after being crushed under the bike. He had cannabis in his system, equivalent to smoking a single cannabis cigarette about three hours prior to death, and was likely to have been affected by the drug. Evidence showed was using the drug to relieve back pain. Inconsistent tyre pressure and the weight of a rear-mounted spray unit were also factors in the death.