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The rugby trophies in the home of Chris Currie's mother tell of a young man who was not only one of the best, but also one of the most fair.

The 20-year-old was an honest man, a responsible boy who avoided the limelight but was always there for his friends, says his father, Wayne.

That is what makes his death so unfair, says Mr Currie.

Around 200,000 drivers would have used the Southern Motorway on Friday, the day Christopher Wayne Currie drove north from Taupo to play in a club rugby match.

He, like most other drivers, would have barely noticed that he was passing under a bridge.

But a twist of fate put his car beneath the overbridge at Otahuhu when someone dropped or threw a jagged concrete slab towards the stream of traffic below.

The result was deadly.

The young building apprentice didn't have a chance. The 8kg piece of concrete smashed through his windscreen and into his head, killing him instantly.

His three female passengers, including his girlfriend Helen, who was sitting next to him, were injured when the 1993 Honda Civic hurtled out of control for 120m into a lamp-post.

Nine metres above them, two young people were seen running from the overbridge.

Other drivers have had lucky escapes.

Police say that after publicity about the tragedy, other motorists called to say that on the same night objects were thrown at their cars.

A team of 25 police are working on the case. If found, the person who committed the "insane act of stupidity" around 9.20pm on Friday could face a manslaughter charge.

Police are hoping that the culprit has left behind incriminating evidence such as fingerprints on the railings, or a distinctive graffiti tag.

They believe the person lives nearby and they will speak at school assemblies today.

Chris Currie was driving north to father Wayne's home in Auckland, preparing to play rugby the following day for the King Country under-20 team.

Instead of greeting his son, Wayne Currie received a call to say Chris had been killed.

"He was doing everything right. There's nothing else he could have done, nothing else," said Mr Currie.

"I would hope someone out there has got a conscience and they're feeling very guilty. They have got to own up to what they have done."

His son's life had been great, he said. He was happy and in love.

"He had been in love with Helen since form one. They took a while to get together, on and off, but they've been together for a few years now."

Chris Currie had a younger brother, Logan, aged 18, and sister, Nicola, 14.

Yesterday, visitors filed into the Taupo home of Chris' mother, Geraldine Vincent, to remember the rugby player who played for both the King Country under-20s and Taupo Sports.

Wayne Currie said his son was a great motocross rider and loved fishing and hunting, but rugby was his passion.

"He never wanted to be the best, he just wanted to take part. He didn't like the limelight, he wouldn't like this," said Mr Currie.

"It will be a big funeral. I don't think he knew how popular he was."

Family friend and King Country under-20s manager Doug Boughton said Chris was not an outspoken boy but he was a leader whom others looked up to.

He had taken his own son Todd under his wing as well as other younger rugby players around Taupo.

"He was very good to my boy. I thanked him when I went to see him yesterday. I guess it's a bit late now," Mr Boughton said.

He said that when Chris was not organising his team he was helping out with the local Taupo Nui-a-Tia College 1st XV rugby team.

"His death has not set in with a lot of the boys yet," Mr Boughton said.

Coach Justin Marsh said the team did not play on Saturday. The game was called off because of Chris' death but they had decided to dedicate the rest of their season to their mate.

Friend Peter Dampney said Chris was an "old soul" for 20. He had also looked after his son Sam.

"When he turned 18 and could drink it was Chris who took him into town and made sure he was all right and got him home again. That's what he was like," Mr Dampney said.

Last Sunday, Papakura bus driver Bill Hart's bus windscreen was hit by stones dropped from the same overbridge. He was taking a group of 30 elderly Papakura people for an afternoon trip into central Auckland. He saw three youths on the bridge.

"As we approached, I thought they were up to something. They were leaning over the bridge. I was very apprehensive as we went through."

The bus was undamaged. He called 111 and reported the incident from the Britomart Transport Centre.

A police spokesman said yesterday that in response, officers on the motorway were alerted and motorway surveillance cameras scanned the bridge. But Mr Hart's call was 30 minutes after the incident and no one was seen on the bridge or in the vicinity.

Detective Senior Sergeant Neil Grimstone said police were confident they would catch the person responsible for Mr Currie's death.

"It doesn't appear to be the work of a criminal group. It's perhaps a random act of stupidity that you would expect juveniles to be involved in."

Police yesterday door-knocked in the area and were going through information that had been phoned in, said Mr Grimstone.

"The phones have been running red hot. A lot of people are talking to us.

"An act of stupidity has cost the life of a fine young man who was innocently driving to a rugby game with friends.

"If you were in the group and you didn't throw the rock, I suggest you would want to be giving us a phone call. If one of your mates threw it, you would be wanting to distance yourself pretty quickly."

He did not know where the concrete had come from, but noted the construction sites near the overbridge.

Contractors are building new motorway ramps; mounds of fill are being shifted.

One of the sites is fenced off from the footpath, but at another, a pile of old concrete was visible inside a gateway secured with a single chain strung across it.

Mr Grimstone said the surveillance camera high above the overbridge had been no help as it was pointing along the motorway rather than down at the bridge.