Proposed mandatory sentencing laws for serious assault have been watered down to leave out less serious assault offences in response to criticism their inclusion would be a "bridge too far".

Fairfax Media understands the NSW cabinet has given approval to the second tranche of mandatory sentencing laws as part of its response to alcohol-fuelled violence.

The first set of laws introduced last month applied a mandatory minimum sentence to an assault resulting in death.

The second set of new laws, expected to be introduced in the NSW Parliament as early as Tuesday, were originally proposed to apply to all assaults, including minor ones.

This proposal was heavily criticised by the legal profession as being too broad and opening the way for people serving mandatory jail sentences for minor assault charges.

The government had originally proposed a four-year mandatory sentence for affray, two years for assault causing bodily harm and two years for assaulting a police officer.

But legal experts in Australia, the US and Canada warned the laws would fill jails at a high cost to the public purse while being unfair to vulnerable groups, including indigenous people.

Instead, the government is expected to introduce minimum sentences of three to five years for the more serious categories of assault, including assaulting a police officer and reckless assault resulting in grievous bodily harm.

Offences will need to have been committed in public places and be fuelled by alcohol.

Fairfax Media understands the new set of mandatory sentencing laws would apply to offences under section 34 (1) to (4) of the NSW Crimes Act and to section 60 (3).

A government source said: "Serious assaults will be dealt with in a serious way.

"The idea that the laws covered any type of assault was a bridge too far."

Premier Barry O'Farrell last month introduced a minimum mandatory sentence of eight years for assault causing death if the offender was intoxicated. Legal experts say this is effectively an eight-year non-parole period and the sentence would be around 11 years as a result of sentencing guidelines.

NSW Greens MP David Shoebridge said Premier Barry O'Farrell's approach to mandatory sentencing policy making was "amateurish and shows he clearly had no understanding of the impact of his original announcement".

"He is pressing on with mandatory sentencing when all the research shows it doesn't deter crime. It is intended to pacify the media and not fix the problem," he said.

It is understood the Department of Premier and Cabinet has been driving the new laws and was forced to admit its original proposal went too far after anomalies identified in other countries, including the US, were brought to its attention.

The push in NSW for mandatory minimum sentences to combat alcohol-fuelled violence has come as similar laws in North America face a backlash for being too harsh and only serving to fill jails.

A coalition of US senators, including Tea Party Republicans, has been campaigning to slash mandatory sentences for some drug crimes, saying the laws are outdated and have led to overcrowding of prisons.

In Canada, successful legal challenges have been made against mandatory sentencing for firearm offences on the basis it was unconstitutional.

Anthony Doob, from the centre for criminology and sociolegal studies at the University of Toronto, said Canada's federal government had been using mandatory minimum sentences "to market itself as being tough on crime".

University of Ottawa law professor Elizabeth Sheehy said the US experience had shown mandatory minimum sentencing laws were a "failed experiment" that had no impact on reducing crime.

"US politicians who led the charge for such laws regularly recant and come to Canada to warn us off the track," she said. "Some states teeter on bankruptcy because they simply cannot afford the investment they have made in the prison system."

Professor Sheehy said mandatory minimum sentences had been used as a political solution to particular crime problems. "Mandatory minimums have their harshest impacts on vulnerable populations and have a devastating effect on our efforts to de-carcerate indigenous peoples," she said.

David Brown, emeritus professor of law at the University of NSW, said: "The US is currently grappling with the damaging effects of mandatory sentencing policies enacted in the 1980s and 1990s, and attempting to wind them back.

"We stray down the road to mandatory sentencing at great peril and for little or no gain, save as a political salve and media distraction."