Army helicopters worth almost $4 billion cannot open fire when troops are abseiling from the aircraft because the doors are too narrow.
Defence has tried three different designs of gun mounts to overcome the problem but none have fixed the issue.
"It is not an issue of the gun mount design. It is an issue of the width of the door," Defence official Shane Fairweather told a Senate estimates hearing on Tuesday.
"The door isn't wide enough to enable the safe exit while firing is taking place."
Defence chief Angus Campbell acknowledged there was an issue with gun doors on the Taipan choppers.
"We know it, we are working on it," he told the committee.
But General Campbell said the Army had reworked its tactics so troops could still reach the ground while pilots opened fire on enemies during dangerous operations.
Helicopters are deployed in pairs so one can shoot while troops rappel from the second aircraft.
"It requires a careful planning of the employment of our aircraft. We never fly an aircraft on its own," he said.
Almost $3.8 billion has so far been spent on the multi-role helicopters to replace existing Sea King and Blackhawk fleets.
The Howard government signed the contract for the first of the choppers in 2005.
Australian Associated Press