Bruce Honore died of a severe head injury after being hit by a motorcycle while watching his grandson race, a coroner has ruled.

The 68-year-old Waikato farmer was at the Cambridge Motorcycle Club in May 2016 to cheer on his 8-year-old grandson when the tragedy unfolded.

Honore was watching the junior riders' track with his wife Merilyn and daughter Tanya from a grass strip that separated it from the main track at the motocross event in Leamington.

A 20-year-old rider on the senior track accelerated at the bottom of a hill to get through a boggy patch before letting the throttle off as he approached a corner at the top, coroner Gordon Matenga said.

"The motorcycle hit the outer berm causing [the rider] to become dislodged from the motorcycle and he landed on the track."

For an unknown reason the accelerator on the motorbike remained engaged and under high revs, and it continued on a straight path up the berm, over the fence becoming airborne and striking Honore, his wife and another man.

"Mr Honore received a fatal blow to the head and despite immediate medical assistance, died at the scene as a result of the injuries he sustained," Matenga said in his decision released today.

"I once again express my condolences to the Honore whanau."

Meri Honore was also injured in the incident and was taken to hospital.

At the time Tanya Honore said her father was a proud family man who died before he could meet his two newest grandchildren, who were due to born shortly after the freak accident.

"He was just always there. He was a quiet but an amazing dad, husband and grandfather. He was fantastic with the grandkids. He was a very gentle person."

Her parents had been married 43 years and had three adult children.

Tanya Honore said her parents did everything together and her mother had been left shocked at losing her husband so suddenly.