A hearing into the powers of the ACT Firearms Registry has been told government contractors "probably" used illegally modified weapons during recent kangaroo culls in Canberra.

The acknowledgement came during the ACT Civil and Administrative Tribunal (ACAT) appeal of Canberra-based gun manufacturer Gareth Crook, who was blocked from building sniper rifle prototypes for interstate police.

Mr Crook's company was approached to design, test and potentially supply the high-powered rifles for use by special operations groups in 2015.

However, in January the ACT Firearms Registry rejected the company's request to fit the rifles with suppressors and collapsing stocks, both of which were required by the brief but are illegal in the ACT.

During Mr Crook's appeal he argued that contractors engaged by the ACT Government during recent kangaroo culls had been allowed to use suppressors on their weapons, and that he should be allowed to as well.

The argument prompted the registrar's lawyer to acknowledge "no-one appreciated at the time ... that the use [of suppressors] may not have been, and was probably not, in accordance with the law."

The lawyer noted the registrar had reviewed the legislation and planned to recommend changes to allow for the use of suppressors for "proper and legitimate purposes".

ACAT President Geoffrey McCarthy said while he sympathised with Mr Crook's position, the ACT Firearms Registry was right to reject his application.

However he noted that under the changes the registrar would push for, it was likely Mr Crook's application would be permissible.

Government 'took AFP advice'

Daniel Iglesias, the director of Parks and Conservation with the ACT Government, said before each annual cull took place, parks consulted with the AFP, of which the ACT Firearms Registry is a part.

"As part of our planning, every year before we commence a kangaroo cull the parks service goes and speaks with the AFP ... and what we do is we say 'Ok, what are the licensing arrangements that need to happen?"

"We rely on the advice we receive from the AFP to ensure that what we are using in the field is appropriately licenced and authorised."

Mr Iglesias said it was not until preparations were made for this year's cull that the AFP told parks that suppressors were not legal.

"This particular year, the AFP advised that we'd require an exemption to use suppressors," he said.

"It would appear as if in retrospect, we needed exemptions last year, but neither ourselves as the applicants, or the AFP were aware of that."

Push for legislative changes

Mr Crook said he had been in touch with the office of Minister for Police and Emergency Services Simon Corbell, and was pushing for legislation changes to allow both himself, and kangaroo cull contractors to use suppressors.

"You've obviously got to ... try these things and actually work out whether its an administrative decision process or whether it is actually a legislative issue," he said

"As far as suppressors go, the Minister's already issued a regulation to that effect until legislation is potentially changed.

"It actually does give an interim solution to both culling contractors and firearms dealers and manufacturers."