The political motivation of the arsonist "does not provide a basis for leniency", Justice David Mossop said.

Nicholas Malcolm Reed arrived 20 minutes late to the ACT Supreme Court on Wednesday, when he was sentenced to eight months behind bars.

After this time in jail, the custodial part of Reed's sentence is set to be suspended upon him entering into a two-year good behaviour order.

Reed, 32, was found guilty of arson by a jury during a trial in September.

Co-offender Bruce Shillingsworth jnr was found guilty of aiding and abetting arson.

The pair were involved in the December 2021 fire that caused $5.3 million worth of damage to the historic Canberra building during a series of anti-government protests.

On Wednesday, Justice Mossop said Reed had deliberately started the fire to damage the doors, but "he was reckless to the more substantial damage of the building".

On December 30, 2021, Reed used a wooden shield to scoop coals from a nearby fire and walked them to the doors of Old Parliament House.

He repeated this multiple times.

In one social media video played to the court during the trial, "Reed can be seen putting a bundle of sticks on top of the fire," Justice Mossop said.

"At that stage the fire was burning vigorously."

When Australian Federal Police officers tried to ascend the steps to the building, "protesters linked arms and formed a barrier to prevent police access," the judge found.

"Because of the resistance of the protesters and in order to prevent a more violent altercation ... police were directed to retreat."

During this interaction, Reed grabbed an officer's radio, ripped it from his vest and pushed him backwards.

The December 2021 protests were in relation to the past and present treatment of Aboriginal people, and for Indigenous Sovereignty.

On Wednesday, Justice Mossop said that "while the political motivation of the offending is clearly a consideration ... it does not provide a basis for leniency".

"The boundary between acceptable and unacceptable public protest must be maintained," he said.

Justice Mossop said a pre-sentence report painted "an unusual picture" of Reed.

The report found the 32-year-old "attempts to linguistically distract others" from his academic and emotional shortcomings.

"It was difficult to understand the points [Reed] was making," Justice Mossop stated.

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Giving evidence last month, Reed told the court his actions "caused a lot of harm" to the Indigenous sovereignty movement.

"In particular, in the way it has been looked on and the perception afterwards damaging a lot of work a lot of people have done for a long while," he said.

Reed will be sentenced for a charge of obstructing a territory public official at a later date.

Shillingsworth is set to face court again in February 2024.

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