This is a Pacific Black Duck, Anas supercilious, and is found in most of Australia except for the central deserts. Its habitat is usually deep heavily vegetated waterways such as swamps. At times it will be found in open water and wet paddocks. This particular one was living in an inner city pond, no doubt being fed by the visitors to the gardens. In size it will be between 47 to 60 centimetres.
The following image is one I took several years ago in New Zealand and it illustrates how the Pacific Black Duck forages for food. This is referred to as dabbling. These birds are mainly vegetarian but will supplement their diet with crustaceans, aquatic insects and molluscs.
The Pacific Black Duck is closely related to the Mallard, A. platyrhynchos, a species introduced from the Northern Hemisphere. They have very similar habits and the two species will interbreed.