Ground-handling firm Aerocare is investigating an incident where a clipboard left on an engine cowling was sucked into the engine of a Jetstar plane bound for Sydney.
A union said it was "an unfathomable incident" which Jetstar was also investigating.
The head of E Tu aviation, Kelvin Ellis, said his union was "shocked and concerned to hear about this incident which is unacceptable and should never have happened".
Jetstar says the aircraft with 84 passengers on board had to turn around and return to Auckland 30 minutes into the flight. It has begun an inquiry after the incident on Saturday but would not comment on Aerocare's role.
"The flight took off and was operating normally; the pilots returned to the airport as a precaution, and had a normal landing," said a Jetstar spokesman.