A staff bus is a bus that is driven not in service from the depot to a point on a route for the purpose of changing drivers. This is due to restrictions on how long a driver can drive for, when the limit is half-way through a route, drivers can drive for a longer time without breaking restrictions. This also reduces the distance a bus has to run not in service.
There are many stops where this is done on the Sydney Buses network. There is a full list here provided by user Stu on Bus Australia forum.
As seen on the list, there are some cases where a staff car is used for this purpose as there are space restrictions.