Swimmer Nick D'Arcy could face a maximum two-year jail sentence over an assault which led him to be dropped from the Australian squad for the Beijing Olympics.

D'Arcy has pleaded guilty to punching Simon Cowley at a Sydney bar in 2008.

The attack left the former Olympian with five metal plates in his face.

At D'Arcy's pre-sentence hearing in Sydney, the Crown said intoxication and poor impulse control led to his crime.

Earlier, D'Arcy's lawyers said Cowley had slapped the swimmer just before the attack and had then moved towards him to shake his hand.

It was then that D'Arcy punched the former Olympian.

D'Arcy's lawyers argue the attack was not premeditated and it was possible he thought he was defending himself.

He will be sentenced at the end of March.

'Proving his manhood'

Magistrate John Favretto said it was clear D'Arcy had little self-control when drinking, but was not necessarily using excessive force when he punched Cowley.

"He is a young man, a very fit young man," he said during sentencing submissions in Sydney's Downing Centre Local Court.

"It is not that he necessarily used excessive force ... sometimes it's how the punch is thrown.

"Young people in particular seem to need to prove their manhood and they do that physically and violently instead of walking away."

D'Arcy's lawyer, Richard Jankowski, said his client should not be given a custodial sentence because he had already suffered enough.

As well as being booted off the Olympic squad, D'Arcy had endured intense "media harassment", he said.

He had shown sufficient contrition and remorse, pleaded guilty and admitted having a drinking problem requiring treatment, Mr Jankowski said.

"His real problem has been consumption of alcohol followed by a change of attitude," he said.

"Other than that there's nothing on his record or nothing in his history that suggests he would offend in any other way."

Mr Jankowski said D'Arcy needed to undergo treatment for his drinking problem to minimising the risk of reoffending, and a custodial sentence was not necessary.

"He should be allowed to continue to develop and to mature and to fulfil that potential that clearly is demonstrated in his curriculum vitae, together with what is expressed by his referees," Mr Jankowski said.

Mr Favretto was also asked to consider not recording a conviction, as it could jeopardise D'Arcy's future swimming career overseas.

However, prosecutor Peter Aitken argued D'Arcy should be jailed, then given a supervision order and forced to give up alcohol altogether once released.

But Mr Favretto said a total alcohol ban would be setting D'Arcy up to fail and could damage his chances of rehabilitation.

"Consistent with maturity and rehabilitation, the ability to consume alcohol is a proper social part of ordinary life and there's absolutely nothing wrong with it," he said.

"It could be detrimental to his rehabilitation."

Mr Favretto said taking control away from someone who was being treated for an impulse control problem was not a good idea.

D'Arcy left the court flanked by his lawyers and declined to talk to waiting media.

- ABC/AAP