But lawyers for the activist, Christiaan Klootwijk, 70, insist he was using the whistle to alert shooters to what he thought was a wounded kangaroo, which humane shooting protocols required be put down before the cull could continue.

Closing arguments were delivered by both sides in the Klootwijk case in the ACT Magistrates Court on Thursday afternoon, following a protracted hearing.

Klootwijk was found to be blowing a whistle near the boundary of a southside nature reserve during a cull operation on July 1 last year.

He had allegedly been warned previously about being near the cull site in late June, when police say he was using loudspeakers to play a bugle-type noise.

But his defence, led by barrister Jack Pappas, argued there was no evidence that the shooters had proper firearms licences or a licence to cull kangaroos, meaning the court should consider the operation unlawful.

Mr Pappas also argued that Klootwijk was blowing the whistle to warn of a possible wounded kangaroo.

The cull operation is bound by a strict code for the humane shooting of kangaroos and wallabies, which states that if a kangaroo is injured, but not killed, then the cull must stop until the animal is found and put down.

Mr Pappas said that Klootwijk, far from hindering the public officials, was helping them to do their job properly and in accordance with the code.

"Even if Mr Klootwijk was there as a protester... what he did in this instance was not to hinder, because that's what everyone was required to do upon becoming aware of the reasonable possibility of an injured animal," he said.

When first approached, Klootwijk told a ranger he could hear a screeching sound and was concerned about a possible wounded animal. The ranger also heard the screeching and went to investigate.

But prosecutor Sarah McFarland argued that it was obvious that Klootwijk was there to disrupt the shooters, something that was cemented by comments to police in an interview, in which he expressed a desire to prevent the deaths of the kangaroos.

"This is a place that he should not have even been at the time," she said. "His only motivation to be there was to interrupt the cull."

Ms McFarland also rejected any notion that he was blowing the whistle to identify himself to shooters.

"He was dressed in dark clothing, he had a head torch on that was not lit, and he was ... standing in bushes," she said. "He knew exactly what he was doing, that he wasn't blowing that whistle to identify himself."

She said Klootwijk's actions directly caused the shooters and the operation leader to be idle for 45 minutes, doing nothing.

Chief Magistrate Lorraine Walker has reserved her decision on the case, and is expected to hand it down later this month.