NSW sentencing changes to cost 'millions'

Mandatory minimum sentences for alcohol-related assaults in NSW will put extreme pressure on the prison system and cost taxpayers hundreds of millions of dollars, the Greens say.

Following intense public pressure over alcohol-fuelled violence, Premier Barry O'Farrell announced a raft of new measures on Tuesday to crack down on offenders.

Among the proposed new laws are increased maximum sentences and the introduction of mandatory minimum sentences for serious and fatal assaults involving drugs and alcohol.

Greens MP David Shoebridge said the sentencing changes would lead to the NSW prison population swelling by at least 50 per cent.

Explainer: O'Farrell's new measures against alcohol violence

Comment: Hasty policy won't solve alcohol-related violence

According to NSW Bureau of Crime Statistics and Research (BOCSAR) figures obtained by the Greens, almost 4700 people in the state were convicted of serious assault offences in the 12 months to September 2013.

A further 900 people were convicted for assaulting police.

Of these, police figures reveal almost half the serious-assault convictions were alcohol-related, and more than 60 per cent of the assaults on police involved alcohol.

Based on these figures, Mr Shoebridge said the prison population would increase by about 2500 people within a year of the legislation coming into effect.

After that, it would rise even further.

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To accommodate the increase, spending on adult prisons would increase from about $750 million a year to more than $1.1 billion, he said.

"The costs of this policy, both socially and in direct budget terms, are enormous," he said in a statement on Wednesday.

"Over the five-year budget cycle, this grossly flawed scheme will cost NSW taxpayers well over $2 billion on a policy that we know will fail."

He said the money would be better spent on schools, public transport and hospitals.