A schizophrenic man who launched a campaign of telephone harassment from behind the walls of mental institutions will spend no more time in jail.

The ACT Supreme Court has also heard Jeremy Dash-Greentree thought he was taking revenge on a paedophile when he doused the front door of a flat in petrol and set it alight.

Justice Hilary Penfold on Wednesday sentenced the 30-year-old to 22 months, suspended after a little more than 18 months on entering an eight-month good behaviour order.

But Dash-Greentree had been locked up on remand intermittently since his arrest on the arson charge in October 2009, and was released from custody having already served his time.

The mentally ill man is medicated and will remain in the care of the Brian Hennessy Rehabilitation Centre for some time, where he has ''settled in well''. But mental health workers fear he may be incapable of living unsupported in the community.

In 2006 Dash-Greentree was hearing voices in his head which he thought could be cured by assaulting someone, the court heard.

It is alleged he believed one voice belonged to a sex offender who abused him as a child. He would later say he had not been molested but maintained the man was a paedophile.

In June 2006 Dash-Greentree went to a flat in Griffith and set fire to the doormat, causing more than $2600 damage, but he wasn't initially arrested.

In 2007 he surfaced in Western Australia, turning himself in and confessing to both the Griffith arson and another arson crime in WA.

He was sentenced to 18 months and in 2009 was released from custody and transferred into a psychiatric facility.

From inside the facility he began repeatedly calling a Belconnen florist, making 29 calls in one month, sometimes threatening and sometimes suggesting they start a relationship.

Dash-Greentree returned to the ACT and continued contacting the woman, in one instance showing knowledge of her movements the previous day, down to the score of her child's soccer game.

He was arrested on the arson charge and taken to the Canberra Hospital's psychiatric services unit, where he continued to contact the woman.

The defendant pleaded guilty to arson and using a carriage service to harass, with offences of using the postal service to menace and stalking also taken into account.

The court heard Dash-Greentree had been diagnosed with paranoid schizophrenia, post-traumatic stress disorder and had spent 10 years living on the street.

He had a significant criminal history but his stint behind bars in Western Australia was his only time served.

Justice Penfold was satisfied Dash-Greentree's moral culpability was reduced because he was motivated by false beliefs stemming from his mental illness.

She said it was appropriate in the circumstances for him to serve out his sentence in the community.