The most common cause of retaining wall failure in Australia is inadequate sub-soil drainage behind the wall.
For example, after periods of rain, even short periods, the build up of sub-surface ground water can be massive, and if it has no where to drain to it can build up behind retaining walls , causing them to lean and become unsafe over time.
This problem can also be made worse by the use of poorly draining soils behind the wall such as clay.
When built correctly, a retaining wall will have a sub-soil drainage system behind it , usually by means of an Ag Pipe, or a series of Ag Pipes, that will collect any moisture below the ground surface and direct it to the properties stormwater drainage system. Large retaining walls can also have penetrations, or drainage points, built in across the wall to help relieve water pressure.
If you have any concerns about the condition or appearance of any retaining walls, a licenced Building Inspector can provide advice and feedback on your best course of action.