Rhyce Bailey Butcher Corney, then aged 18, was in Dunsborough for school leavers celebrations on November 20 last year when his friends and another group became involved in a dispute.

Butcher Corney claimed he was startled by another teenager, who suddenly appeared to the side of him, and took a swing, the West Australian District Court heard on Monday.

Prosecutor Katie Kemm argued an immediate prison sentence was appropriate for causing grievous bodily harm and general deterrence should be a significant sentencing factor.

But defence counsel Linda Black said her client, who has played football for the Swan Districts colts, pleaded guilty and had good prospects.

"He was startled by the sudden appearance by the victim," she said.

"He had no idea where exactly he was striking him."

The court also heard Butcher Corney was on bail for this offence earlier this year when police saw him throw a bottle in the direction of his girlfriend following an argument.

It resulted in him spending almost three months in prison until he was eventually fined $500 for the crime.

Ms Black said Butcher Corney was terrified in prison.

"He looked like he was about to pass out," she said.

"The biggest fear for him is going back there."

Ms Black urged Judge Stephen Lemonis not to impose an immediate prison sentence.

"Mr Butcher Corney has had a wake up call of his life by virtue of what has occurred," she said.

Butcher Corney will be sentenced on Tuesday.

Australian Associated Press