Another witness, who was first on the scene, said she saw the bus driving "relatively fast" before the crash.
"It came along from Smales Farm, along the road from the side of the motorway," she said.
The bus was not stopping for passengers so was driving at a decent speed when it hit the fence.
"It just went straight into the barrier," said the woman, who had just disembarked from her bus when the accident happened.
The fence "sort of went inside the bus and crumpled up".
The male driver had slight injuries and was taken to hospital by ambulance, she said. One police car also attended.
A police report initially said the bus had also hit a pedestrian, but the woman witness said there had been no pedestrian involved.
Auckland Transport said buses were running at reduced frequency due to the crash and emergency services were at the scene.