"It appears from the initial investigation that she has taken a shortcut up a grassy incline and the quad bike has hit a small lip hidden in the grass and flipped the bike on top of her and pinned her down, possibly suffocating her."
Police were called to the scene just after 3pm.
The matter had been referred to the coroner and Worksafe NZ, he said.
A Worksafe NZ spokesman confirmed an investigation into the incident was under way but initial findings suggested the woman was using a quad bike on sloping terrain when the bike rolled on top of her.
Waihi Beach fire chief Ian Smith said his brigade was called to the scene although he could not attend.
He believed the woman was a St John volunteer who was known to the St John first-response team who attended the accident scene.
Mr Smith said he had been led to believe the accident happened in the morning but the contractor was not found until the afternoon.
Craig Thompson of Athenberry Orchard, where the accident took place, declined to comment on the incident but said it was being investigated and the woman was not an employee of the orchard.
A spokesperson for Agfirst Bay of Plenty, the woman's employer, confirmed there had been "an unfortunate fatal accident".
"Our thoughts are with the family," he said.
