There are many different types of master key systems in Australia. However, in general, a master key can open all the locks in a system and each lock has its own specified key or keys which work under the master key.
This works on many different levels. For example, in an office building where a company has several offices, each needing their own key, employees are given access to their office only, while the master key opens all the locks and is held by the manager or owner, security guards and sometimes the cleaners. Master key systems can also be used in schools, your own home, or by landlords.
The design of the master key system can include covering companies whose offices are on multi-levels of a building. In this case, a grand master key is provided for a manager or owner to gain access to all doors on all floors. A master key is then provided for each individual floor and on each floor, every office has its own key which cannot be used to enter other offices.