"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.
Emergency services on site at Akoranga Station due to an incident involving a bus. Bus services are still operating,but at reduced speed.^SB pic.twitter.com/3fNFr5U4CZ