An armed robber has told the ACT Supreme Court he said please and tried not to point his replica gun at anyone when he robbed the Fyshwick Post Office.

Dean Vaughan, 37, has pleaded guilty to the crime and said he did the robbery to get money for drugs.

He told the court that once inside, he tried not to threaten anyone directly.

"I walked up to the counter, I had the gun but did not point it at anyone. I said please," Vaughan said.

Vaughan did accept staff would have been frightened even though it was only a replica gun.

"I am not trying to take myself away from the fact I have done an armed robbery," he said.

Prosecutor Tanya Skvortsova quizzed Vaughan about other armed robberies.

"Did you say please on previous occasions when you robbed businesses?" Ms Skvortsova asked.

Vaughan replied: "I do not know."

Vaughan told the court since the crime he had re-evaluated his life, after the trauma of trying to save his cellmate using CPR, before the man died in May.

"After my cellmate passed I've realised how precious life is," Vaughan said.

"I do need to change, I really do."

His lawyer John Masters said it showed he was capable of putting others' needs ahead of his own when he performed mouth to mouth on the man until medical help arrived.

"He put the priority of the health of his cellmate over the dangers to his own health," Mr Masters said.

Vaughan told the court he is planning to do rehabilitation programs in prison, and will go to stay with his parents, who are both prison wardens, in New South Wales when he is released.

He will be formally sentenced tomorrow.