Pilot David Turnock allegedly threatened to fly a stolen plane into the Sky Tower because his wife had left him and he wanted her back.

The actions of the Auckland flight instructor forced the evacuation of 500 people from the Sky City entertainment complex on Saturday night after he stole a Piper Cherokee and threatened to crash it into the Sky Tower.

Instead the 33-year-old crashed into the sea, just off Kohimarama Beach, injuring himself and causing major disruption on election night.

Mr Turnock, a former flight instructor at Massey University's School of Aviation, stole one of the school's planes from Ardmore Airport about 7pm.

He flew towards Manurewa and drew the attention of traffic control staff at Auckland International Airport. Police said he made the threats against the Sky Tower through air traffic controllers.

As Mr Turnock flew towards the city centre, Sky City staff started evacuating the building and emergency service agencies planned for the worst.

A police helicopter monitored the flight but police could do nothing to prevent the plane from heading towards its destination.

Mr Turnock got as far as the city centre but it is understood he never got close to the tower and instead headed towards Kohimarama Beach.

Friends Glen Spittle, 33, and James Debenham, 34, noticed the plane when it was about 15 to 20m above sea level.

They watched as it did a u-turn, got lower and lower and eventually went wing-first into the sea.

The men swam, fully clothed, towards the plane, which was all but submerged. They found Mr Turnock disoriented and bobbing in the water about 10m from the plane and helped him ashore.

"When we sat him down on the sand that is when the shock must have really hit him and he just started shivering," said Mr Debenham.

Mr Turnock was taken to Auckland City Hospital in a serious condition and was yesterday interviewed by police.

Detective Inspector Gavin Jones said two investigations were under way, one to determine whether the pilot was criminally liable and one by the Civil Aviation Authority.

Mr Jones said the police learned about the threats through air traffic controllers and indicated the police would obtain the tapes of conversations with the man, which might give a picture of the man's demeanour. The police had no direct contact with the man.

People within aviation circles yesterday said Mr Turnock had recently returned to New Zealand with his wife but that she had left him.

His wife, who is also a flight instructor, last night told the Herald she could not comment on the allegations because she was still talking to police.

Countdown

7pm: Piper Cherokee taken from Ardmore.

8.15pm: Auckland International Airport warns police about unidentified plane over Manurewa.

8.30pm: Police warn Sky City of threat.

8.45pm: Entrance to Sky Tower closed, Sky Deck, Observation Deck and Sky Lounge cleared.

9pm: Sky City warned to also clear restaurants.

9.15pm: Entire Sky Tower cleared. 500 people evacuated between levels 44 and 63.

9.40pm: Plane crashes into sea off Kohimarama Beach.