The following description was taken from OTSI: Bus Safety Investigation Report – Bus Wheel Separation :

The Office of Transport Safety Investigations (OTSI) is an independent NSW agency whose purpose is to improve transport safety through the investigation of accidents and incidents in the rail, bus and ferry industries. OTSI investigations are independent of regulatory, operator or other external entities.

Established on 1 January 2004 by the Transport Administration Act 1988, and confirmed by amending legislation as an independent statutory office on 1 July 2005, OTSI is responsible for determining the causes and contributing factors of accidents and to make recommendations for the implementation of remedial safety action to prevent recurrence. Importantly, however, OTSI does not confine itself to the consideration of just those matters that caused or contributed to a particular accident; it also seeks to identify any transport safety matters which, if left unaddressed, might contribute to other accidents. OTSI’s investigations are conducted under powers conferred by the Passenger Transport Act 1990. OTSI investigators normally seek to obtain information cooperatively when conducting an accident investigation. However, where it is necessary to do so, OTSI investigators may exercise statutory powers to interview persons, enter premises and examine and retain physical and documentary evidence.

On the morning of 22 October 2014 a State Transit Authority (Sydney Buses) Mercedes-Benz bus lost a left rear wheel on a passenger service between Chatswood Station and Parramatta Station. There was only one passenger on the bus at the time, and no injuries resulted. The investigation determined that the loss of the wheel was the result of fatigue failure of the wheel retaining bolts, due to the wheel nuts having been inadequately tensioned and checked as part of a scheduled service three weeks earlier.

Contributing to the wheel loss was the inability of a daily visual check of the bus to detect loosening wheel nuts. Furthermore, a visual check of the bus when rough riding was noticed did not detect a wheel bolt that had broken earlier. Sydney Buses immediately investigated the servicing of the bus, and found strong indications that recent servicing involving the wheels had not been properly performed. Sydney Buses took appropriate action to prevent recurrence.