Father admits shooting dead Vic son, 30

Ex-truck driver Peter John Smith used an unregistered firearm to shoot and kill his adult son just a week before Christmas.

The 70-year-old on Thursday in the Victorian Supreme Court pleaded guilty to murdering Andrew Thomas Smith, 30.

He also admitted possessing unregistered rifles and a shotgun after the fatal shooting at Mulgrave on December 18.

Justice Andrew Tinney said he would seek more information about what led to the shooting, including why the victim allegedly threw a frying pan at a stove.

Defence barrister Richard Edney asked that Thursday's plea hearing be adjourned to allow time for Smith to undergo a neurological assessment.

The case was "confronting" and so Smith's plea needed to be heard in its entirety, Mr Edney said.

About 15 family members sat in court, some in tears, one saying she felt sick as they left.

The case will return to court September 21.