In August last year, jurors found Bruce Shillingsworth jnr, 32, guilty of aiding and abetting the arsonist who lit fire to the front doors of what Justice David Mossop described as a "significant national institution".

That decision received cries of "no!" from a dismayed public gallery, with one angry supporter pointing angrily at the jury.

The December 30, 2021, blaze, a destructive result of anti-government protests, caused $5.3 million worth of damage.

"Mr Shillingsworth was clearly the leader of the protest," Justice Mossop said on Friday afternoon.

"He was therefore influential in the conduct of others."

The judge found Shillingsworth was involved in helping create and foster the fire by encouraging someone to paint over a CCTV camera, telling protesters to form a shoehorn around the growing blaze and forcefully preventing police from getting to it.

Shillingsworth was handed a wholly suspended jail sentence of 18 months. He was also fined $8000 and ordered to enter into a three-year-and-six-month good behaviour order.

The extended requirement for good behaviour, Justice Mossop said, "will ensure any social activism on his part will remain within the guardrails of a peaceful democracy".

The judge said he was "significantly influenced" by the prosecution not opposing a non-custodial sentence.

The protest group in question included now-convicted arsonist Nicholas Malcolm Reed, aged in his early 30s.

Defence barrister James Sabharwal had earlier asked the court to draw a distinction between the roles each man played.

"It's not the same ... [Shillingsworth] himself did not touch or light the fire," the barrister said.

Reed was found guilty of starting the fire with hot coals and stoking it into a destructive blaze. There was no evidence of pre-meditation or planning with others.

Reed was sentenced to one year and 11 months in jail, which the judge elected to suspend after eight months behind bars.

Justice Mossop said Shillingsworth had "demonstrates limited remorse for his conduct" and "he appeared to regard Old Parliament House as a legitimate target".

"Mr Shillingsworth is undoubtedly entitled to be an agitator," the judge said.

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"He is entitled to express his political views, loudly, in a manner that's annoying to government officials."

However, the judge said political motivation did not provide basis for leniency.

On Friday morning, Shillingsworth took to the witness box to answer questions about his offending.

"We had no intention of causing any harm to any individual," he said.

"It wasn't the intent to come to Canberra, of the ACT, and to burn down the door.

"That institution was the one that made those policies to enact what we still see today. Nothing has changed, hence why we were there."

Shillingsworth had also said spending time behind bars could be "the start of a generational cycle".

"It tends to be a snowballing effect," he said.

"If something was to happen, in terms of me being incarcerated, there's a high likelihood my children, being as young as they are, it will start a cycle."

The court heard Shillingsworth has 15 years of youth work experience, and "that's not to mention what we do in the community".

Shillingsworth was also sentenced for charges of defacing public property, assaulting a territory public official and obstructing a territory official.

He was fined $300 and given a 12-month good behaviour order for those offences.

The man previously spent two days in custody for his crimes.