Stephen Michael Jones, 55, of Braddon, will be sentenced in April on one count of committing an act of indecency in the presence of another person.

An ACT Supreme Court jury took less than two hours on Tuesday to find Jones guilty of the incident in February last year.

A cleaner spotted Jones walking along Elouera Street while she was vacuuming the front steps of the Braddon building about 10.20am on February 12.

She told the court he then pulled his penis out of his shorts, took a seat on a bench, and started to masturbate.

He yelled out, ''Hey,'' when he spotted her and pointed at his open mouth, she said.

Her phone had run out of credit, so she went across the road to the Salvation Army office to call the police.

During cross-examination, defence counsel Richard Davies asked her about the accused's other activities while seated on the bench.

The woman said Jones had also taken off his bag, pulled out a beer and lit a cigarette - all the while holding his penis in his right hand.

When asked if that activity was physically possible, the woman replied: ''I could see very clearly. I'm telling you what I saw.''

The court also heard from a Salvation Army employee, who called the police at the woman's request.

The jury heard the employee had a clear, uninterrupted view of Jones while he sat on the bench.

He said the offender had a hand in his crutch region and a bottle of beer in the other hand at the time.

But he told jurors he did not see the man's penis or him masturbating.

Jones, from the witness box, said he had a beer bottle between his thighs while seated on the bench and the woman had been ''gravely mistaken'' in her evidence.

He said his baggy shorts and underwear made masturbating in the manner alleged a ''physical impossibility''.

In cross-examination, prosecutor Sara Gul said Jones saw the cleaner and took his chance to masturbate in front of her.

''You took your chance to engage in some shocking behaviour,'' Ms Gul said.

Jones replied: ''No, I did not.''

Justice John Burns ordered pre-sentence and drug and alcohol reports be readied for the April sentence.

The judge continued bail for Jones.