As a general guideline, it is always a good idea to get in contact with your local council. They will tell you the city requirements for either building or renovating along with any specifications to be applied.
Many city councils require their residents to gain approval and permits for the building of new structures, structural alterations to the foundation of the building or extensions. Additional permits may be required from specific councils depending on the area and specifications of the proposed decking.
Some councils require permits for all decking regardless of height or size of the structure and additional development applications if the decking is raised above certain levels.
A general rule to follow when adding decking is; you will need council approval if the decking is raised either equal or greater than 500mm from the existing ground level. The proposed area is further than 900mm from the property line or exceeds a height greater than 2.4m. The permits are necessary to ensure the safety of the structure along with abiding by regulations and specifications concerning privacy and interference with neighbouring properties.
Depending on the area you are residing in there can be additional approvals concerning materials, termite treatment, requirements and necessary standards for areas prone to bushfires.