Occupational Health and Safety experts said the valve of the fire extinguisher was damaged when it fell over - or was knocked over - and the sudden discharge of high pressure gas sent the cylinder spinning, then flying.

The cylinder spun around several times on the ground before becoming airborne in the south Taranaki tragedy.

It punctured the lower leg of a 54-year-old man while on the ground then flew through the air, hitting the woman in the head and smashing through a corrugated plastic window high in the wall.

Ambulance officers tried to save the woman, but she died at the accident scene.

It is understood she was walking into a mechanical workshop at Uhlenberg Haulage in Eltham when the accident occurred.

Taranaki District Health Board ambulance manager Ian May said the woman was in a critical condition when paramedics arrived, and they tried unsuccessfully to resuscitate her.

The man, whose injuries were described as moderate, was taken to the emergency department at Hawera Hospital.

OSH regional service manager Brett Murray said it was still not clear whether the fire extinguisher fell over or was knocked over.

He said OSH inspectors had investigated the scene and would interview people who witnessed the bizarre accident over the next couple of weeks.

"They're in a state of shock. It's a family business. So we'll be working on that once people get over the initial shock and grief of the incident," he said.

OSH will also test the fire extinguisher.

Mr Murray said he thought it could be weeks before OSH know the exact cause of the accident.

Police, who are also investigating the incident, said they were satisfied it was merely a tragic accident.