A bruised and shaken Helen Clark admitted she wondered if she would die when an aircraft door flew open over the sea near Wellington yesterday.

With confidential security papers flying through the cabin, and as two police officers tried to grip the door's tiny handle and force it shut, the Prime Minister was tossed in her seat, severely bruising her lower arm.

She was working in the back of the chartered twin-engine Piper Aztec when turbulence forced open the door when the aircraft was 8000ft above the Kapiti Coast.

Asked what went through her mind, Helen Clark said: "Are you going to live or die? What goes through your head is you hope you're going to land."

She said there was a sudden noise and the plane plummeted. Despite having her seatbelt on, she was thrown upwards and crashed down against a window sill, hurting her arm.

"I could feel that was a bit painful, but then I noticed a struggle going on with the door which had blown open, and two policemen that were with me were struggling to hold onto the door, so that was quite dramatic.

"I don't like to think what might have happened if I'd been sitting in the co-pilot's seat, trying to struggle on my own."

The Mountain Air pilot took the plane into a controlled, steep spiral as one of the two diplomatic protection squad officers rummaged in his briefcase for a truncheon, which they slid through the door handle for them to grip.

Constable John Burridge and Senior Constable Dave Reid spent the next 15 minutes clinging to the door, against the strength of the wind and turbulence, until the plane landed safely at Paraparaumu Airport.

The pilot had sent out a mayday call but the aircraft was on the ground before emergency services could deploy rescue boats off the Kapiti Coast.

Helen Clark and her press secretary, Kathryn Street, the only other passenger on board, still looked shaken when they spoke to reporters several hours later. "It was shocking. It really was quite shaky for us," the Prime Minister said.

On landing, she declined to kiss the ground Pope-style but said she did send a text-message to her parents to tell them she was all right.

Her injured arm was x-rayed at the Kapiti Medical Centre, and found to be badly bruised but not broken.

She blamed shock on her initial decision to keep reading despite the drama.

"I said to Kathryn, 'Thanks for the papers', watched the police struggle and then thought I'll just keep on reading the security report on Gallipoli - which actually didn't seem quite as dangerous as what we were doing at the moment."

Helen Clark praised the professionalism of the pilot - who had flown her last week during fog. She chartered this flight on Tuesday evening, again because of fog. She also said it was fortunate the two police officers were there. "I'm really proud of them."

The Transport Accident Investigation Commission said it was too early to identify a cause. John Mockett, chief investigator of accidents, said it would be about six months before the commission's findings were released.

But Mountain Air owner Keith McKenzie said the fault - a split rubber grommet that caused the "one in a million incident" - had been found. The aircraft was repaired and was already flying again.

Mr McKenzie said he knew it was the "13th" when he heard his aircraft had been involved in an incident with the Prime Minister on board.

Helen Clark said later that the incident had not put her off flying, but she would not be drawn on whether it lent weight to arguments that the Prime Minister should have a dedicated aircraft.

- Additional reporting: NZPA