Blake Watt, 24, punched the man once in the face after he made mention of a person named "Bevan" during a conversation with Watt and two other men during the chance encounter at the Shellharbour service station.

The punch left the victim with fractures to both sides of his jaw, which required surgery the following day.

The man was unable to work and forced to eat and drink through a straw for six weeks.

In a sitting of the NSW District Court in Sutherland on Monday, Watt's lawyer, Elizabeth Parkes, said the dispute between her client and the victim stemmed from a belief the victim was "standing over" the person named "Bevan", understood to be Watt's friend.

"The injured person was involved in an earlier altercation with one of Mr Watt's friends," Ms Parkes said.

"Mr Watt admits he was under the influence of alcohol and other substances at the time of the incident."

Watt was sentenced to two years' jail in Port Kembla Local Court earlier this month, with a non-parole period of 18 months, but appealed the decision believing he'd been dealt with too harshly.

Presiding Judge Paul Conlon agreed to take three months off Watt's non-parole period owing to the fact it was his first time in jail.

However he said the Local Court magistrate had been justified in imposing the overall sentence of two years' jail for the charge of recklessly causing grievous bodily harm.

Watt is eligible for parole in April next year.