The father of the young man killed by a concrete block thrown from a motorway overbridge says the justice system has let him, his son and his family down.

Wayne Currie said the four-year prison term given to the teenager who killed his son Chris Currie, 20, in the High Court in Auckland today was "pathetic."

Ngati Reweti, 15, was given a year off for his early guilty plea to manslaughter and his remorse, and a year off because of his youth when he appeared for sentence before Justice Helen Winkelmann.

He was originally charged with murder but a jury found him not guilty and convicted him instead of manslaughter.

The charge came after he threw an eight kilogram block of concrete from an overbridge on Auckland's southern motorway on August 19, last year.

It smashed through the windscreen of the car Mr Currie was driving and slammed into his chest, probably killing him instantly.

After the sentencing today, Wayne Currie said the family was disgusted at the four-year sentence.

"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 had hoped to send 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."

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."

In court today Reweti's lawyer Lester Cordwell said he was extremely remorseful but Mr Currie rejected that.

"I have had no contact from him or his family and with children's actions I hold their parents responsible."

He said for that reason the expressions of remorse meant nothing.

In court today the judge said throwing the concrete block off the bridge was an act of "breathtaking stupidity.

"Your mindless act took the life of a young man," she said.

However, she said it was a spur of the moment act and she did not accept the crown submission that it was premeditated.

Crown prosecutor Aaron Perkins said the sentence should deter others from committing such acts and people should be able to travel on the roads without the fear of a missile being thrown and the consequences of that.

The judge said Reweti's family had said he was a gentle, loving son but the deputy principal from his school had a different view and said he was a challenging student who was frequently absent from school.

He had smoked marijuana and drank alcohol the day he threw the block off the motorway overbridge but he had accepted full responsibility and had not blamed the alcohol or drugs.

The judge said Chris Currie's mother had not made a victim impact statement because it was too painful.

The judge said she was satisfied six years was an appropriate starting point but he would get a year's discount because of his early guilty plea to manslaughter and his remorse, and a year because of his youth.

She said there would be no minimum term imposed on the four-year jail term.

- NZPA