Mark Andrew Johns, 53, was living on the ground floor of the complex in Belconnen at the time of the blaze on May 29.

In the ACT Supreme Court on Friday, Justice Michael Elkaim said Johns was drinking heavily, "perhaps due to emotional upset", on the day in question.

"In the early evening, no doubt well affected by the alcohol, he set fire to a beer carton inside his unit," the judge said.

"The fire spread rapidly and the offender went outside."

By the time firefighters arrived to extinguish the blaze, it was already "well-established".

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Johns tried to go back into the building but was restrained by one of the firefighters.

He told the firefighter and police officers, who soon arrived on the scene, that he was responsible.

All but one of the eight units in the complex sustained damage from either the fire itself or smoke from it.

Johns was arrested at the scene and has been in custody ever since.

He pleaded guilty to an arson charge in August.

Sentencing him on Friday, Justice Elkaim noted that the former apprentice boilermaker had suffered "a very serious head injury" at work.

The judge said this had led Johns to start drinking up to 14 stubbies of beer a day, describing him as "a significantly mentally disabled person".

In light of Johns having already spent more than five months behind bars on remand, Justice Elkaim decided to impose a partially suspended jail term with rehabilitation as its focus.

He ordered that the balance of a backdated 12-month jail sentence be suspended from November 12, giving Johns' father and other people involved with the man a week to arrange suitable accommodation for him ahead of his release.

Johns will be subject to an 18-month good behaviour order when he goes free.

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