The government conceded last year that the use of silencers in the annual kangaroo cull was "probably not in accordance with the law", which bans noise suppressors.

The illegal possession or use of silencers attracts a fine of $30,000 and up to two years in jail.

But a Justice Directorate spokesperson said the shooters had not acted recklessly because they were acting under a permit issued by the registrar of firearms.

"It would be difficult, if not impossible, to prove the fault element of either offence that is recklessness" under the Criminal Code, the spokesperson said.

The registrar of firearms had issued the permit "in error" but that was an administrative error, not a criminal offence.

For the director of public prosecutions to take action it must be satisfied that the criteria to prosecute were met, particularly the public interest test, and that the cost was justified, the spokesperson said.

The firearms register is the deputy chief police officer.

Asked what action police had taken about the illegal use of silencers, ACT Policing said, "No investigation has been undertaken by ACT Policing as the previously issued permits by the registrar of firearms were found to be incorrect and as such, this does not constitute a criminal offence by person acting under those permits."

To deal with the permit problem, the firearms registrar issued declarations every three months in 2016, authorising the use of silencers for the cull and for firearms dealers to manufacture, repair and test firearms on their premises.

Animal Defenders Officer director Tara Ward said while that probably made silencers legal, the first of the declarations was also flawed and arguably did not legalise the use of silencers.

She also pointed out that not only were the permits illegal, in 2016 the cull licence was also invalid, according to the Magistrates Court, in a finding upheld in the Supreme Court.

Despite that finding, the government pushed ahead with the 2016 cull.

The series of errors raised questions about the government's management of the entire cull, she said.

"Our concern is that if the government is getting this wrong, what else are they getting wrong? Should we be asking questions about the whole science behind the cull? This legitimately raises those questions."

Ms Ward said the government's suggestion that recklessness could not be proven was open to debate. If the shooters came from NSW, they could be expected to be aware of the need for a permit. If there was no reference to silencers on the gun licences, that "arguably could have alerted a responsible shooter to the need to inquire whether a permit to use the silencers was required".

The justice spokesperson said shooting was the most humane method, the cull was regularly audited by vets, and the marksmen were expert.

Suppressed firearms were used because they had less recoil "which promotes shooter accuracy, protects the shooter from hearing and nerve damage, and minimises noise disturbance".

The illegal use of silencers since 2009 was uncovered when firearms dealer Gareth Crook challenged the refusal by the firearms register to give him a permit to test rifles for supply to the Victorian police. Last year, Mr Crook went to the ACT Civil and Administrative Appeals Tribunal, where the registrar argued he had no power under the law to issue permits for the use of silencers.

Mr Crook pointed out that they had been issued for many years for the kangaroo cull. In response, a lawyer for the registrar "acknowledged the inconsistency, but explained it on the basis that no one appreciated at the time suppressors were used to conduct the kangaroo cull that the use may not have been, and was probably not, in accordance with the law".

The tribunal ruled in favour of the registrar for firearms, agreeing he did not have the power to issue the permit.

The Supreme Court threw out the conviction of a protester for hindering the cull on the basis that if the cull was not valid then it could not have been illegally hindered.

Government shooters have killed close to 11,000 kangaroos and hundreds more joeys since 2009.