Why Australia have two court systems?

Answer 1

It is not entirely correct to say that Australia has two court systems as each court does fit within the one court hierarchy. There are different levels of courts which do exist with in either a state or federal jurisdiction. These two jurisdictions mean that certain courts can deal with certain matters in original and appellate jurisdictional matters. These two jurisdictions (state & federal) are created typically through the Constitution which divides powers between the two main levels of government. Each government must follow these divisions and create laws only within their power to do so... otherwise it will be invalid. These different laws (state and Commonwealth) fall into certain court jurisdictions.

FOR EXAMPLE:

Any Constitutional matter will be heard in the High court as they are responsible for such matters of federal importance. District courts will administer only state laws (as will courts below and including the Supreme court in the hierarchy).

Sorry for such a complex answer... but the law is complex, and as a student of this profession we learn to answer with detail that allows for greater depth and understanding.