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.

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

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.