Matthew Kelly, then 45, was walking his usual route to the Hopkins River Milk Bar to buy a drink on the afternoon of June 16, 2020, when he was violently assaulted by a 35-year-old man he'd never met.

Trevor Van Kempen, 38, previously of Bates Road, was observed near the Hopkins River Bridge twice punching the victim to the head shortly after 4pm.

Mr Kelly fell to the ground before Van Kempen repeatedly punched him to the face using both fists, kicked him and jumped on his head.

Van Kempen pleaded guilty in Warrnambool County Court on Tuesday to intentionally causing serious injury in circumstances of gross violence.

The court heard a witness watched as Van Kempen continued to assault the victim despite being incapacitated.

Mr Kelly, who lives with an intellectual disability known as Fragile X syndrome, had not long been dropped off at the milk bar by his disability support worker.

He was later observed on the ground, lying flat on his back and with blood leaking from his ears and mouth.

A passing motorist pulled over and upon tooting his horn, watched Van Kempen walk away from the scene.

An off-duty police officer was alerted to the assault and the offender was located near Warrnambool's Simpson Street.

Van Kempen initially denied the offending before repeatedly apologising, stating it was "because of my trauma".

Mr Kelly was transported to Warrnambool Base Hospital where he was placed in an induced coma.

He was then flown to Royal Melbourne Hospital where he underwent brain surgery.

He spent six weeks in hospital recovering from fractures to his face and skull, bleeding on the brain and several significant complications, including pneumonia and seizures.

In a victim impact statement, Mr Kelly's sister and guardian Elizabeth Murphy said the assault permanently impacted her brother's physical and emotional health.

She said Mr Kelly used to walk five kilometres a day but could no longer manage more than 500 metres.

He sustained loss of his sensory abilities, memory loss and could no longer take part in his favourite activities, Mrs Murphy said.

Mr Kelly's brother Patrick said his brother had since lost his energy and desire to leave home.

He said his brother was physically weak, had poor balance and lives with pain he can't properly articulate.

"A small piece of independence that saw him make his way to his local milk bar for a drink was taken away from him," he said.

Maria Kelly, another sister of the victim, said her brother had lost his spark, emotional curiosity and capacity to socialise following the "intentionally cruel and vicious act".

Sally Buckley, representing Van Kempen, accepted the offending was unprovoked but referred to a psychological report which said her client heard the victim yell and act aggressively after misperceiving a threat.

She said at the time of the offending, Van Kempen was in a heightened state of paranoia after being subjected to attacks in his share house.

Ms Buckley said on one occasion, a portion of Van Kempen's ear was bitten off, and he was in fear of being seriously assaulted again.

She said Van Kempen suffered bipolar, complex trauma and psychosis.

Ms Buckley said her client had pleaded guilty at the earliest opportunity, was remorseful and often became "extremely tearful" when talking about what had occurred.

Van Kempen, who has served more than 620 days in custody on remand, will be sentenced at a later date.

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