Helen McCreadie had not seen her boyfriend, Chris Currie, for a week.

She had been on holiday in Rarotonga and had plenty to tell him as they drove along the Southern Motorway in Auckland on Friday night after meeting at a service station nearby minutes earlier.

Their happy reunion ended abruptly when an 8kg piece of concrete thrown from an Otahuhu overbridge hurtled through the windscreen of the 1993 Honda Civic, killing Chris instantly.

The car hurtled out of control for 120m before hitting a lamppost.

Still in shock yesterday, Helen McCreadie could not bring herself to describe those moments on the motorway.

"It just hurts," she sobbed.

"I can't believe they took something so precious away from me."

The Taupo couple, both aged 20, met at intermediate school, and Helen still wears an inexpensive - but infinitely precious - ring on her finger from that time.

"It was the first one Chris gave me back in Form 1."

More recently, when the couple went to shopping malls, Chris used to "test" her, she said.

They would pause at jewellery store windows, and he would point out rings, asking her preference.

"He was a sweetie," she said. "He was irreplaceable. That [his killing] just doesn't happen to people like Chris. We had so much planned for the summer."

Whoever was responsible for ending Chris' life "just cut it way too short".

Comforted by friends Femke Johnson and Lisa Wi, Helen wept: "I love him, and I am going to miss him. He was my boy."

On Friday, the couple had met near the home of Helen's aunt, not far from the Princes St overbridge in Otahuhu. They were to spend the weekend in Auckland with his dad, Wayne Currie, so that Chris, an apprentice builder, could play in a rugby match on Saturday.

"We were so happy to see each other," Helen said.

Because Chris was unfamiliar with a more direct "short-cut" to his father's place, he drove back to the motorway to head into Auckland City.

With two of Helen's cousins in the back seat, the couple were chatting excitedly and swapping news when the lethal blow fell about 9pm.

Helen, a Taupo social science student, managed a smile as she told how she and one of her cousins had met Chris at the service station to show him the way to her aunt's place. The other cousin had travelled from Taupo with him.

Police yesterday upgraded the investigation into Chris' death to a homicide inquiry after talking to a witness who saw a person hunched over, carrying the concrete slab towards the Otahuhu overbridge just before Chris' car was hit.

Detective Senior Sergeant Neil Grimstone said a description of that person would be available today.

"I'm quite happy that that person is the person who threw the concrete from the overbridge."

Earlier yesterday, Wayne Currie publicly challenged the person responsible for his son's death to "face up to what you have done and be accountable".

"If you think you were tough enough to do that, be tough enough to admit your actions."

Holding his raw emotions in check, Mr Currie said: "It wasn't an accident. I think Christopher deserves justice for what happened."

Urging the culprit to come forward, he added: "I hope they have a huge guilty conscience at the moment. I hope it is eating them up.

"That person has done so much damage, it is unforgiving in my eyes.

"That person has broken up a lovely family unit and upset a lot of people."

Mr Currie, who had gone to Raglan the previous weekend to watch Chris play rugby for the King Country under-20 team, said his elder son had been a well-adjusted young man, happy in his job and in love with Helen.

He had given his parents "no grief whatsoever" and would be "hugely missed" by younger brother Logan, 18, and sister Nicola, 14.

"They looked to him for guidance and support."

The large, close extended family - who had coped with the deaths of other members from cancer and an accident - were strong, Mr Currie said.

But nothing could bring back Chris, who lived life "almost to the rulebook".

"I'm not just saying it because he's my son, but he was a good boy. This has left a huge hole in many people's hearts."