St Vincent’s Hospital head of emergency Gordian Fulde said there had been hardly any severe brain injuries caused by alcohol-fuelled violence in the six months since stronger laws, including mandatory sentencing, were introduced in February.

The dramatic drop in severe alcohol-related injuries came after the doubling of a prison sentence given to Kieran Loveridge on Friday for the one-punch killing of Sydney teenager Thomas Kelly two years ago on Monday.

“The absolute decrease in severe head injuries has been spectacular and terrific,” Professor Fulde said.

“We’ve definitely seen a very marked decrease in as far as we’ve had hardly any severe brain injuries from alcohol-fuelled violence.”

Loveridge’s original sentence sparked public outrage and prompted the government to bow to pressure and implement strict mandatory sentencing laws for violent offences fuelled by alcohol.

Professor Fulde said he believed an increased police presence, intense media scrutiny and the legislation had helped change people’s attitudes.

But he said he would hold his breath to see if the “magic” transformation of his emergency department continued into summer and the “silly season”. “That will be really when we will know where we are at,” he said.

On the eve of the two-year anniversary of Mr Kelly's death, his father said Loveridge's increased sentence and the decrease in severe alcohol-related injuries at St Vincent's was what his family had been waiting for.

"We’ve literally spent the past two years of our lives trying to change things," Ralph Kelly said. "That children can go out more safely today than what they could do six months ago is good news for everyone."

He said he believed the work done by the Thomas Kelly Youth Foundation and legislative changes had taught young people there were consequences for their actions

"We always believed that to get behavioral change there had to be legislative changes made so that there were consequences," he said.

Monday marks two years since his son's life was taken by one, swift and pointless punch.

"Any anniversary is difficult but probably the hardest is his birthday," he said.

Law Society of NSW president Ros Everett said it was measures such as high-visibility policing in Kings Cross that had led to the fall in violent incidents, not the mandatory sentence for one-punch assaults.

‘‘I would love to see the one-punch law repealed because mandatory sentencing can lead to very unfair results,’’ she said.

Ms Everett said the decision of the Court of Criminal Appeal to increase Loveridge’s sentence from five years to 10 years showed mandatory sentencing wasn’t needed.

‘‘It is proved by this case that the system is working. There are built-in safeguards in the system we have. Mandatory sentencing brings in really unfair results. You can’t have one size fits all.’’

Ms Everett was critical of ‘‘knee-jerk reactions’’ by governments to heinous crimes that led to changes in the crimes or bail acts.