An eyewitness account has led Auckland police to step up an inquiry into the death of a motorist.

Christopher Currie died on Friday night, when a concrete chunk smashed through his car's windscreen as he drove up the southern motorway.

Police now say they are dealing with a homicide inquiry.

Detective Senior Sergeant Neil Grimstone says an eyewitness account has shed some new light on the incident.

The eyewitness has reported seeing a person who was stooped over, carrying a piece of concrete from a nearby construction site towards the bridge.

Detective Sergeant Grimstone says he has no doubt that this is the person responsible for dropping the concrete onto the car.

He adds that as part of the investigation police want to speak to juvenile street Otahuhu gangs, whose graffiti tags are in and around the bridge.

Meanwhile, speaking in Taupo this afternoon, Chris Currie's father, Wayne, has pleaded for the killer to come forward.

He hopes who ever did it has a huge guilty conscience and that it is eating them out.

Mr Currie believes his son deserves justice for what has happened as it was not an accident.

He says if the person was strong enough to throw an eight kilo block of concrete off an overbridge they should also be strong enough to come forward and be accountable.

Wayne Currie says it is a living nightmare for him and his wife, although they are trying to focus on Christopher's brother and sister.

Chris Currie will be buried at 11am on Wednesday in Taupo.