In Perth, the soil quality that lawns are grown on tend to be quite poor. Applying a good fertilizer helps ensure your lawn obtains the right amount of food to keep it healthy.
As a general rule of thumb, apply fertilizer every 8 weeks. This will enable it to be fed a good amount. However, if slow release fertilizer is used, apply less frequently. Reading the back of the packet will also give you some idea as well. Applying too much fertilizer will increase the nitrogen and acidic levels in the soil so do not over fertilize.