The trial of Shane John Mason, 50, continued in the ACT Supreme Court on Wednesday, following a four-month break.

Mr Mason, who has a long history of mental-health issues, is accused of lighting two fires in Yarralumla in early January 2014.

The fires were noticed by passers-by, who called triple-0, and they were extinguished.

After the first fire, Mr Mason was found hiding in long grass, with a cigarette lighter, a hunting knife, and cannabis in his possession.

After the second fire, 11 days later, he was found in water beneath a bridge. He had sustained serious burns and was flown to a Sydney hospital for treatment.

That fire was lit during a total fire ban.

Mr Mason was sleeping rough in bushland at the time, and had recently spent time in a psychiatric treatment facility.

There are two issues in the trial: whether Mason lit the fires – which he denies – and if he did, whether his mental impairment meant he did not know the wrongfulness of his actions.

The accused gave evidence on Wednesday afternoon, denying knowledge of lighting either fire. He said he became aware of each while in the area and had tried to put them out.

Prosecutor Anna Jamieson-Williams used cross-examination to ask Mr Mason why he had decided to put the flames out.

"Well, who wants a bushfire? Of course it's the right thing to do," he said.

Ms Jamieson-Williams responded: "So it's the wrong thing to do to light a fire?"

Mr Mason said: "Definitely, and I think very poorly of people who do that."

He agreed that he would have known it was wrong at the time of the fire.

Earlier, Mr Mason told the court he tried to put out the second fire using his Driza-Bone jacket, but there was an explosion, which set his body alight.

"It was burning. It was alight, the whole arm, and a little bit on my back and my face, so I rolled," he said.

He said he ran to the river to douse the flames, and then saw police arrive.

The Crown accused him of giving varying versions of his story to a police officer, a lawyer, and two psychiatrists.

He allegedly told a police officer he might have thrown a cigarette butt on the ground, but now says he cannot remember that conversation.

It was also put to Mr Mason that he had stolen a dog from the RSPCA a day or two before the second fire, despite telling police at the scene that he had had the dog for weeks. Parts of a multi-tool in Mr Mason's possession were found inside the cage from which the dog was stolen.

After initially telling the court he found the dog chasing kangaroos, Mr Mason conceded he might have stolen the animal and could have lied to make sure it wasn't taken away from him.

"I don't know. I could have done that, but I don't remember and I don't know."

The judge-alone trial continues before Justice Hilary Penfold on Thursday.