"The truck came down the expressway and some object was tossed from the bridge, which hit the centre of the windscreen of the truck," he said.
"Because it hit the centre of the window, the glass that shattered on the inside missed the driver. If it had hit in front of the driver, then the driver would have more than likely crashed."
Shelford said it would have been "very frightening" for the driver, who is now back at work.
"I suspect, if it's young people or teens, that they don't really have a full understanding of the consequences of tossing those objects at vehicles or off the bridge."
He also said it could be somebody "disgruntled" with heavy vehicles, or vehicles in general.
Shelford said it was lucky the objects did not hit the load of cars the truck was carrying, as the load cost "in the millions".
The truck had to go to the panelbeaters for damage to its body, on top of the windscreen damage.
A police spokeswoman said the objects were being thrown for about 15 minutes between 2.15 and 2.30, and drivers in both directions were being affected.
She urged anyone witnessing similar dangerous behaviour to call police immediately.
Anyone with information on this incident is asked to email Constable Vinton of the Kapiti Police on BVDX33@police.govt.nz or send police a private message on Facebook. They can also call Crimestoppers on 0800 555 111.
A 15-year-old boy was jailed in 2006 for throwing an 8kg slab of concrete from a construction site near the Princes St overpass onto Auckland's Southern Motorway, killing Taupo man Chris Currie.
The rock went straight through the windscreen, glanced off Currie's chin and struck his chest, killing him instantly.
Ngatai Rewiti was 14 years old when he committed the crime, for which he received a manslaughter conviction and a four-year prison sentence.
