If you're landscaping and need to install a retaining wall, like any other development, most will require Council approval. However, under certain provisions you may find your retaining wall could be classed as an exempt development providing they are not higher than 600mm from natural ground level, are not within one metre of a boundary and comply with the conditions of the State Governments ‘Exempt Development Code’.
If you do need to lodge a Development Application via the Planning Portal, Council requires that where retaining walls are located close to boundaries, that the associated drainage pipes and backfill are located entirely within the respective allotment, and should not exceed 900mm if retaining cut, and 600mm if retaining fill as shown in the diagram below. Also, the weight of soil behind a retaining wall can create massive hydrostatic pressure, in some cases, causing poorly designed walls to collapse. For this reason, any retaining wall over 1000mm from natural ground level requires engineers certification be submitted with an application for a construction certificate which can be lodged via the Planning Portal.