Teenager Ngatai Rewiti, jailed for four years yesterday for throwing a slab of concrete from a motorway overbridge that killed a motorist, could be free by Christmas.

He could also get home detention within weeks.

Rewiti, now 15, appeared for sentencing in the High Court at Auckland yesterday for the manslaughter of Taupo man Chris Currie.

Mr Currie, 20, died instantly when Rewiti threw an 8kg hunk of concrete from the Princes St overpass on to the Southern Motorway in August last year.

The slab slammed through the windscreen of Mr Currie's car, striking him on the chin then crushing his chest.

Rewiti is unlikely to spend much time as a sentenced prisoner.

Under current law, anyone sentenced to a term of more than two years need only serve one-third of the sentence before becoming eligible for parole.

Time spent on remand is counted.

Rewiti would be eligible for parole after 16 months, but as he has already been in custody for 13 months he could be released by Christmas.

A back-end home detention application - available to prisoners within three months of their scheduled release date - would be available to Rewiti already.

Sentencing Judge Helen Winkleman said Rewiti's actions displayed "breathtaking stupidity", but she accepted he felt remorse for the killing.

She said his showing-off to school friends was the reaction of a 14-year-old "trying to come to terms with what he had done".

Crown prosecutor Aaron Perkins had argued that Rewiti's age should not be too much of a factor in sentencing.

"This prisoner has taken the life of another young man in disturbing circumstances," he said.

"He is old enough to know right from wrong ... he knows about the realities of flesh and blood."

Rewiti's actions were "brazen", with limited premeditation, and any sentence handed down must reflect that, Mr Perkins said.

The youngster had thrown the concrete in an attempt to "achieve notoriety" with his friends, to whom he later bragged about the killing.

"Absolutely no concern was shown by the prisoner, after the rock was dispatched.

"Rather, he was making statements to his friends in the days that followed, statements that were quite contrary."

A starting point between six and 10 years would be appropriate, he said.

Rewiti's counsel, Lester Caldwell, had urged a starting point of five to seven years in prison.

He said Rewiti was remorseful about what had happened, he just had trouble showing it.

"He is not a person who wears his heart on his sleeve ... any [previous] expression of emotion has been punished. For him to come out with such an expression of remorse is significant."

Justice Winkleman finally accepted six years was a suitable starting point, then granted Rewiti a two-year discount. She granted a request by the defence that he be allowed to serve his time in a youth justice facility.

A bitter Wayne Currie, Chris' father, said after sentencing that the justice system had let him, his son and his family down.

He called the four-year term pathetic and said he had hoped it would have sent a message that "criminal behaviour would not be tolerated - send that out as a deterrent and hopefully we won't have so many people in jails".

"We feel let down by the justice system. I feel the judge has disrespected us as good people and good people all over New Zealand," Mr Currie said.

He said his son was killed by the sort of person he would have tried to help.

"I think he would be let down. He would be pretty disgusted in the justice system. Four years is pretty pathetic.

"At the end of the day this guy picked up a large concrete block, walked up the hill, dropped it off the bridge and killed our son," Mr Currie said.

He rejected Rewiti's claim of remorse. "I have had no contact from him or his family and with children's actions I hold their parents responsible."

- Additional reporting NZPA