The Airline Pilots Association is calling on Air Nelson to prove it is not doing pilots out of a job by using Air Force staff.
Two RNZAF pilots have been seconded to the airline since February to work as co-pilots for 18 months, in order to give them extra experience in the use of computerised cockpits.
The training will help them fly Orions, Hercules and Boeing aircraft.
The Airline Pilots Association is concerned trained pilots are missing out on work because of the scheme.
Association president Mark Rammell says Air Nelson, a subsidiary of Air New Zealand, refuses to show the association evidence it can operate without the airforce staff.
But Air New Zealand says the airforce pilots are additional to the full pilot complement at Air Nelson.
Air Force spokesperson Squadron Leader Kavae Tamariki says the alternative would be to send the pilots overseas for expensive training in flight simulators.
Squadron Leader Tamariki says the Air Force is looking at smarter ways of getting officers trained and the scheme is working well for both organisations. The RNZAF will be getting new flight simulators, he says.