Time for action: Hard decisions ahead for NSW Premier Barry O'Farrell.

Under pressure to take action to reduce alcohol-related assaults ... NSW Premier Barry O'Farrell. Photo: Edwina Pickles

Fatal ''one-punch'' assaults where drug or alcohol use is a factor will attract a mandatory minimum sentence of eight years in jail under controversial new laws to be announced by the state government after the deaths of teenagers Daniel Christie and Thomas Kelly.

Under pressure for weeks to respond to increasing community concern over alcohol-fuelled violence, Premier Barry O'Farrell will announce the proposed new laws on Tuesday as part of a broader policy response, it is understood.

Died after punch: Thomas Kelly.

Tragedy in Kings Cross .. Thomas Kelly died after being punched.

A senior government source confirmed a mandatory minimum penalty of eight years in jail and a maximum sentence of 25 years would apply to one-punch assaults resulting in death where drugs or alcohol were found to be a factor.

No mandatory minimum sentence will apply for fatal one-punch assaults that do not involve drugs or alcohol. However, under laws flagged last year by Attorney-General Greg Smith and modelled on West Australian legislation, a maximum sentence of 20 years will apply.

This is lower than the 25-year maximum sentence for manslaughter, but the proposed law will make it easier to secure a lengthy prison sentence than a murder conviction, where the Crown must prove the attacker knew the punch would be fatal or cause serious injury.

The NSW government embraced the idea of one-punch laws last year after 19-year-old Kieran Loveridge was found guilty of fatally punching 18-year-old Thomas Kelly in Kings Cross in 2012 and sentenced to a maximum of six years jail.

The sentence was met with outrage in sections of the community and followed the decision of the

NSW Director of Public Prosecutions to charge Loveridge with manslaughter instead of murder after he offered to plead guilty to the lesser charge. Loveridge was jailed in November with a four-year non-parole period.

Demands for the government to reconsider mandatory minimum sentences strengthened after the fatal assault on Mr Christie on New Year's Eve.

The proposed new laws also appear to reflect calls by Mr Kelly's parents, Ralph and Kathy Kelly, for drugs and alcohol to be an aggravating factor in sentencing of those convicted of violent assaults.

The proposed laws appear to represent a significant defeat for the Attorney-General, who in November publicly derided mandatory minimum sentencing as an ''expensive and ineffective crime-fighting tool''. At the time there was speculation the laws were being considered for gun crime after the shooting of a 13-year-old girl in a dispute allegedly involving the Brothers 4 Life gang.

The Australian Lawyers Alliance and other prominent legal figures have criticised mandatory minimum sentences, warning they have no impact on recidivism rates.

The government is also expected to announce measures to achieve better liquor licensing, such as risk-based fees for venues and more policing.