Investigations are under way into how a truck lost two wheels on the Southern Motorway yesterday.
Sergeant Stu Kearns, of the Waitemata serious crash unit, said the truck had recently been serviced but it was too early to say why the wheels came off.
"We are waiting for specialist reports.
Vehicle inspectors will be inspecting the mechanics of the truck, which will then be handed on to CIB," he said.
Greg Miller, group general manager of Toll New Zealand, which operates the truck, said the company expressed its sympathies to those injured in yesterday's accident.
The company was co-operating fully with the authorities and had started an internal inquiry into its operating safety procedures, he said.
Witness Jonathan Sanders was driving south on the motorway. "The rear truck wheels separated and materialised from under the truck trailer, having caused substantial damage to the truck.
"Both wheels separated, with one running to the left of the lanes and the other bouncing madly across the highway into oncoming traffic," he said.
When the wheel hit the windscreen, the bus windows "exploded".
"The wheel struck a car and then the bus. The bus windows just exploded inwards, missing the driver."
One southbound lane was closed and another northbound closed for about two and a half hours.
Some further closures were enforced southbound so police could look for the truck's wheel nuts.
A Vietnamese man was taken to Auckland City Hospital with critical head injuries. Three other passengers were taken to Middlemore Hospital with minor injuries. Everyone on the bus will be offered Victim Support counselling.
Zane Fulljames, NZ Bus general manager of operations, said the driver was severely shaken and had been stood down from his duties so support could be provided for him.
