Jeffrey David Lee bashed Neal Keith Wilkinson, 61, after the pair got into an argument at the front door to the McWhae Circuit house on the evening of March 10 last year.

Police found Mr Wilkinson's body face down near the front door and arrested Lee, 47, at the next day.

Court documents said Lee fled the house after the fight but later went back and took the cash from a pillowcase at the home.

He went into his mother's darkened room while she was asleep and later told her: "Neal's dead, I killed him."

Lee left again soon after. A neighbour heard his mother's cries for help later that evening and called police.

He pleaded guilty in the ACT Supreme Court to murder and theft.

Chief Justice Helen Murrell said Lee's actions in stealing the "significant" amount money had been "callous to the extreme, considering what preceded it".

"It was extremely callous to take that money from his mother in circumstances where he'd just killed his stepfather," she said during Wednesday's sentence hearings.

Lee's barrister, Ken Archer, said an autopsy report revealed Mr Wilkinson suffered a broken jaw and brain injury, but his precise cause of death remained unclear.

It found the victim also had a neck compression injury, perhaps sustained from his position on the floor, which could have contributed to his death, the court heard.

"The extent of the injuries, in a death case, was unusually light," Mr Archer said.

"The difficulty was the extent of the injury wouldn't usually be associated with someone dying."

Crown prosecutor Shane Drumgold, said Mr Wilkinson had suffered heavy blunt trauma injuries caused by several blows to the right side of his face. A scan showed part of his cheekbone had been completely detached.

Mr Drumgold argued Lee had intended to cause his stepfather serious harm when he punched him and those blows had substantially contributed to his death.

He said the fact Lee had left his frail mother at home as her long-time carer lay dead in the hallway before he returned, likely stepping over Mr Wilkinson's body, disabled the phone, nabbed the cash and fled again, made the offences more serious.

Mr Archer said his client, a former public servant and business owner, had started to take drugs including cannabis, ice and methamphetamine after his marriage broke down more than a decade ago.

He occasionally visited his mother and asked for money, some of which he frittered away on gambling and drugs.

Lee was struggling financially and reached his lowest point in the months before the day of the killing, when he got into a "spontaneous" argument with Mr Wilkinson about seeing his mother.

Mr Archer said there was no evidence the attack had been premeditated or planned.

"He did not go there with the intention to argue with his stepfather, let alone to fight or harm him in some way," he said.

He described Lee as "an isolated figure" and said his children had not visited him in jail for six months, nor had his mother been in contact with him since the killing.

Mr Archer said Lee's guilty plea and admissions to his mother showed genuine remorse. His client would now pay the price for his conduct during his time in jail and faced being alienated from his family for the rest of his life, he said.

The court heard a pre-sentence report author found Lee posed a moderate risk of reoffending and was suitable for rehabilitation.

Lee will be sentenced on Thursday.