It was a faulty switch that left an air force plane grounded in Australia and Prime Minister John Key and 80 delegates stranded en route to India.

The Royal New Zealand Air Force said a faulty micro-switch led to a flashing warning in the cockpit of the Boeing 757 - causing the crew to believe the aircraft was not in take-off mode this week.

The system was reset, but the same thing happened when they attempted to take off a second time.

A replacement plane had to be flown from New Zealand to pick the group up, causing them to miss their appointments in Mumbai.

New Zealand First's Ron Mark said the jets fly too few hours in military service, with the 23-year-old Boeing 757s only doing about 571 flight hours each year.

But the New Zealand Defence Force said the faulty switch wasn't a regular fault and had nothing to do with time spent in the air. In fact the aircraft fly between 1000 and 1400 hours every year.

Technicians are now in Townsville working to repair the aircraft.