A cargo plane which got lost on a delivery flight to New Zealand is believed to have had wrongly programmed navigational equipment.

The crew knew they were in trouble when their instruments said they were over New Zealand but all they could see was the Pacific Ocean.

The Convair 580, with a crew of three, was on a delivery flight from Canada and was on its way to Palmerston North when it became lost.

"The crew are somewhat mystified," said Dean Bracewell, managing director of Freightways Express, which will operate the aircraft.

"The plane was going brilliantly, everything was going well, but they had a problem with the navigational gear, which was giving them a position which was incorrect."

The crew knew they were to the east of New Zealand, but didn't know where, Mr Bracewell said.

"They should have been over land, but they weren't."

He said the emergency was believed to have been due to incorrect programming of long-range navigational equipment before the plane left Canada.

The Convair's crew had been relieved to learn the cause of the problem and that human error, rather than technical failure, was responsible, he said.

It appeared the navigation fault became apparent only on the last leg of the flight, from Pago Pago in American Samoa.

The crew reported no problems on the flight from Canada to Honolulu or from Honolulu to Pago Pago.

The two Canadian pilots and New Zealand engineer on board feared they would not have enough fuel to reach New Zealand once back on course, and radioed for help.

Lieutenant Colonel Jeff Richenberger and Major Jeff Puckett heard the weak signal as they piloted their US Air Force jet cargo Starlifter to Christchurch.

Major Puckett, the Starlifter navigator, plotted the Convair's course as missing New Zealand and going on over the ocean.

"They appeared to have no reliable navigational fix ... That's not a good place to be, over the South Pacific," Major Puckett told TVNZ's Holmes programme last night.

The Convair crew could not see land and were flying in bad weather.

The US aircraft intercepted the Convair and escorted it to Gisborne.

Said Lieutenant Colonel Richenberger: "The sound of their voice indicated they were very happy to see our aircraft."

The Convair, due at Palmerston North at 3pm, landed safely at Gisborne two hours later.

It was a long two hours for the wife of engineer Graeme Allan.

Jan Allan, who has been married to her husband for 34 years, went to Palmerston North Airport to meet the plane on Wednesday afternoon.

When it didn't arrive, she went home, not knowing of the difficulties the Convair crew was having.

Her husband's employer, Fieldair, rang later to tell her the plane was having "severe problems".

Mrs Allan then had an anxious wait with a friend and family.

"It wasn't a good time. It's a very personal sort of thing - what goes through your head. I just tried to stay in control."

She got a call half an hour before the plane landed in Gisborne.

"You're still not quite right, but it was a great relief to know he got back."

Last night, Canadian authorities took over from New Zealand to probe how and why the aircraft got lost.

John Mockett, from the Transport Accident Investigation Commission in Wellington, said no one had jurisdiction because the incident happened over the ocean.

"In these circumstances the convention is that the country where the aircraft is registered has first call on an investigation.

"We've offered and been requested to help with information on what happened with the location and return of the craft.

"But the Canadian Transportation Safety Board will handle the investigation."

CAA spokesman Bill Sommer told National Radio yesterday that it was uncommon for an aircraft to lose its way.

"In this day and age it's very, very unusual," he said.

"In days gone by we used to deal in nautical miles in inaccuracies, nowadays we're talking about metres.

"It was some very quick thinking in the National Rescue Co-ordination Centre that allowed them to get the aircraft to fire its emergency beacon.

"Then they used the search and rescue satellite to get a fix on where the aircraft was, and that allowed them to divert aircraft and get an exact position."