A centre of gravity 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 issues with the converted Fletcher plane.
The report says control difficulties for the pilot on takeoff is likely to have stemmed from having skydive passengers too far towards the back of the plane.
The commission will now continue with a more thorough investigation, looking at the design of the converted former top dressing plane.
Investigations will also focus on the type of engine used and whether the passengers should have been wearing safey restraints.
In the meantime, the Civil Aviation Authority has lowered the passenger limit for all similar Fletcher aircraft, from eight to six.