An initial inquiry by WorkSafe is now underway.
Auckland Transport media advisor James Ireland said staff were advised of the incident on Friday and had been told Ritchies Transport was undertaking a full investigation.
"Our deepest condolences go out to family and friends affected by this tragedy."
AT would assist in the investigation, and any other inquiries or investigations in any way it could, Ireland said.
Ritchies Transport has been approached for comment.
The company operates more than 80 double-deckers.
Statistics released earlier this year by AT revealed Ritchies Transport had a disproportionate number of accidents compared to other Auckland operators.
While the company had 260 buses in Auckland, 19 per cent the city total, it was responsible for 48 per cent of accidents, 1 News reported.
More to come