The actions of two air traffic controllers almost allowed a skydive aircraft and a passenger plane carrying 31 passengers to collide south of Auckland last year, say transport investigators.
The skydive aircraft, which had just dropped four parachutists, came within 400 feet, or 122 metres, of an Air New Zealand Bombardier aircraft over Mercer.
A report by the Transport Accident Investigation Commission says both aircraft had been cleared by traffic controllers at Auckland airport.
It says the controller did not fully examine the route the airliner would take when he cleared its destination.
Furthermore, the two-member control team missed an automated collision warning in the control centre.
It says the low level of supervision of controllers could have caused unsatisfactory work habits to develop which may have led to the incident.
Fox Glacier finding
Meanwhile, a distribution-of-weight problem is likely to have led to the fatal crash of a skydiving plane at Fox Glacier.
All nine people on board the plane died in the crash on 4 September.
In an interim report released on Thursday, the Transport Accident Investigation Commission points to problems with the converted Fletcher top-dressing plane.
The report says control difficulties for the pilot on takeoff were likely to have stemmed from having skydive passengers too far back in the fuselage.
The commission will continue with a more thorough investigation, looking at the design of the converted plane.
Investigations will also focus on the type of engine used and whether the passengers should have been wearing safety restraints.
In the meantime, the Civil Aviation Authority has lowered the passenger limit for all similar Fletcher aircraft from eight to six.