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.
Ellis has questioned whether Aerocare's training and split shifts had led to fatigue and contributed to the incident.
He said safety checks broke down at several points around the flight - the clipboard being left there in the first place, the failure to notice it on an engineering walkaround and by a staff member on the ramp wearing the headset who dispatches the aircraft.
Aerocare does ground handling for several airlines in New Zealand and says it has one of the best safety records in this region.
A spokesman said both the leading hand and dispatcher said fatigue was not a factor in the event.
Neither had worked split shifts on the day.
"Aerocare is working with New Zealand aviation authorities, and Jetstar, to understand how this one-off incident occurred.
"Aerocare does not force any employee to work a split shift. Unlike other baggage-handling companies, split shifts are at the discretion of employees and not the company. Any suggestion that these workers were therefore fatigued is wrong.''
Aerocare says it has handled more than one million flights in its 25-year history and has never been fined or sanctioned for any safety incident.
Ellis said there could be under-reporting of incidents, which was common in casualised, low-pay work environments.
Aerocare says its staff in New Zealand are all permanent.