Your licence is a perpetual licence, meaning it does not expire. However to remain current to exercise the privilege you must pass an aviation medical every 1-4 years.
A PPL medical is valid for 4 years if you are under 40 years and over 40 years it is valid for 2 years. A CPL is valid for 12 months only.
You are also required to undertake a flight review every 2 years, which would consist of a cross country flight and some general handling including emergency exercises with an Instructor that is operating under a CASA approved flight school.