A teenage boy who threw a concrete block off a motorway overbridge and killed a man was told by his schoolmates that he was famous.

His victim, Christopher Currie, died immediately when the 8kg block was dropped from the Princess St bridge over the Southern Motorway in south Auckland last August 19.

A 15-year-old boy, who has name suppression, has denied murdering Mr Currie, but his lawyer has indicated the defendant would plead guilty to manslaughter if such a charge was laid.

Young witnesses yesterday told the High Court at Auckland that the rock throwing was a hot topic at the boy's high school the following Monday.

One boy was asked if it was him. "Did you kill that fulla?" he was asked by another.

The accused spoke up, saying "Nah, I did it", the witness said.

"He was saying how he dropped the rock on the car," one schoolboy said.

A friend had seen television coverage of Mr Currie's death, and told the boy he was famous.

The accused asked him what he was famous for. "I said because of the rock he chucked on the car."

Friends filed through the court yesterday, some mumbling, some unable to read their police statements.

One mate, who also has name suppression, told the court that he, the accused and the accused's younger brother sneaked out of their homes about 9 o'clock that evening.

The friend said they wanted to do some tagging as well as go over the Princess St Bridge to the pool hall "to meet girls".

The witness said the accused went down a bank, picked up a "big and lumpy" rock, hid it under his sweater and walked up on to the bridge.

"We were going to gap it [run away] but [the accused] told us to wait 'cos he wanted to hit a car."

The witness said he would keep an "eye out" for cars from the opposite side of the bridge.

"The red car was coming under the bridge and he dropped it."

The boy said they heard the windscreen smash. They then ran off.

The accused's lawyer, Lester Caldwell, asked the witness how long the whole incident took.

He said "no more than a minute" passed between the accused picking up the rock and dropping it off the bridge.

Mr Currie's girlfriend, Helen McCreadie, told the court she was in the red Honda Civic.

At first, she did not know what had happened.

Mr Currie, whom she had known since intermediate school and loved for six months, had just picked her up.

She had been on holiday in Rarotonga, and her friends Rachael Liddicoat and Chanelle Brown were in the back seat.

As they were driving under an overbridge there was a loud smash, Ms McCreadie told Crown prosecutor Aaron Perkins.

Mr Currie's body was limp in the driver's seat, she said.

"Then I looked at Chris. At the time he just looked knocked out. He was just laid back in his seat," she sobbed.

"His hands were by his side, the car seemed to be going faster.

"I tried to grab the steering wheel to correct it but it wouldn't.

"I knew that we were going to crash. I just wrapped my arms around Chris's head."

The car crashed into a lamppost. The petrol tank was ripped from it and burst into flames.

When it stopped, the three women escaped through the rear right door.

Ms McCreadie refused to leave Mr Currie. Ms Liddicoat checked him for a pulse, but the 20-year-old was dead.