A new ''one-punch'' law will carry a maximum penalty of 20 years - double the penalty imposed in Western Australia for a similar law. The proposed law would remove the requirement to prove ''king hit'' assailants knew the punch would be fatal to secure a conviction.

This was the key consideration in the decision by the NSW Director of Public Prosecutions to charge Kieran Loveridge, 19, who fatally attacked Thomas Kelly, 18, with manslaughter instead of murder after he offered to plead guilty to the lesser charge.

Thomas Kelly: Killed by a single punch on a night out in Kings Cross.

Loveridge pleaded guilty to manslaughter and was jailed for a maximum of six years on Friday, with a four-year non-parole period.

NSW Attorney-General Greg Smith said he would introduce a new offence to cover situations where an assault causes death.