But a second charge against Christiaan Klootwijk, 71, was dropped on a legal technicality after the incident in Canberra's south in July last year.

Activists who gathered at the ACT Magistrates Court called out "the cull is the crime, the cull is the crime" as Klootwijk was sentenced on Tuesday after a drawn-out hearing process.

He was arrested after cull workers heard a whistle and yelling soon after they began an after-dark shoot in the Wanniassa Hills Nature Reserve.

The noises sparked concerns a person was on the boundary of the site and prompted a "check-fire", which meant the shooter had to cease firing and unload his gun.

Security personnel spoke to Klootwijk, who explained he believed a kangaroo targeted in the cull had been injured and raised concerns the operation was not being carried out in accordance with the code of practice.

Police were called to the site and the operation was temporarily stopped.

Klootwijk later pleaded not guilty to two charges he hindered a public official during the cull.

During the hearing, prosecutors argued the defendant had hindered the government-contracted shooter by blowing the whistle, and that his actions halted the cull for about 45 minutes.

But defence lawyer Jack Pappas questioned the validity of the shooting licence and suggested the ACT government employees were not hindered by his actions.

Chief Magistrate Lorraine Walker on Tuesday said Klootwijk maintained he had blown the whistle for his own safety and was present "to observe and not to hinder".

He admitted he was there to put pressure on the ACT government over its kangaroo cull policy, she said.

But Ms Walker found it was "implausible" that Klootwijk would use a whistle to alert cull workers to his presence, rather than a head-torch he was wearing at the time.

He had received a previous warning about making noise near the cull site in June, when police said he used loud-speakers to play a bugle-type sound.

Ms Walker was satisfied beyond reasonable doubt Klootwijk intended to hinder the cull and found him guilty of a single offence.

But the second charge was dropped after Ms Walker found the licence was not valid at the time of the incident on a technicality and the contracted shooter hired to carry out the cull could not be considered a public official.

Mr Pappas said his client's behaviour arose from legitimate concerns over how the cull was conducted.

He noted Klootwijk was retired and had limited capacity to pay a hefty fine.

Prosecutor Sarah McFarland acknowledged Klootwijk was passionate about the cause but said his actions had hindered the shooters and extended the operation for 45 minutes.

She said there was a need for general deterrence and for the court to be seen to act swiftly to denounce his behaviour.

Ms McFarland told the court the shooting operation was not an easy or pleasant task and argued shooters should be able to get on with their job unhindered.

In sentencing Klootwijk, Ms Walker said she took into account his age, otherwise good character and his lack of a criminal record.

She found the offence was below mid-range seriousness but acknowledged the need for general deterrence was "a prime consideration".

Ms Walker recorded a conviction and ordered Klootwijk serve a six-month good behaviour order.