The family of Thomas Kelly say the NSW government's plan to introduce "one-punch" legislation will change nothing, and claim the Attorney-General has missed the point on the debate surrounding their son's death.

The O'Farrell government is considering new laws, understood to be modelled on Western Australian legislation, which removes 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 Mr Kelly, 18, with manslaughter instead of murder after he offered to plead guilty to the lesser charge.

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

Afterwards, Mr Kelly's parents, Ralph and Kathy Kelly, called for changes to these laws, and have since been overwhelmed by the public's support for their views.

"How many of our children have to die before somebody does something to change these laws - to make people accountable for what they've done?" Mrs Kelly said.

"If they're never going to be accountable and they get a slap on the wrist, then they're just going to keep offending week after week after week, and somebody else is going to be standing here and be heartbroken.

"No matter what he got, it would never be enough. But four years? It's a joke, it's a joke, an absolute joke and we're horrified, absolutely horrified."

As news of the government's planned laws broke on Monday morning, Ralph Kelly said he did not think the proposal changed anything, because the punishment would still not fit the crime.

"I think the point everyone's missing is [these perpetrators] have killed someone and we need appropriate sentences for that," he told Fairfax Media.

"[NSW Attorney-General] Greg Smith told me 12 months ago that we didn't need one-punch laws because manslaughter had a maximum sentence of 25 years. So what's changed?"

Mr Kelly said he was encouraged that his son's case had sparked a great amount of debate, because there was clearly a need for reform.

However, he said he was unconvinced this was the right step.

His son was struck by Loveridge just minutes after arriving in Kings Cross at 10pm on July 7 last year.

The single, unprovoked blow to the head knocked him to the ground immediately, and he never regained consciousness.