An Act overrules the common law (judge made law) if both apply in the same area. Often an Act adds to an area of the common law, and sometimes Parliament passes an Act that replaces an area of common law completely. Common law that has been replaced may or may not be relevant to the interpretation of the new Act. This may be specifically indicated in the Act or may be determined only after careful interpretation of the Act.
Legislation attempts to control future activity, so legislation is often unable to cover or predict every possible scenario that may arise. A specific case may therefore require a court to decide an Act's meaning in that specific case. The court's interpretation is then read with the Act to make up the law on that topic. For example, the law on families is not contained completely in the Family Law Act 1975 (Cth). It is found in a combination of the Act and decisions of the Family Court made on matters controlled by the Act.
Sometimes an Act may be ambiguous and its interpretation by the courts may be difficult to predict for a particular case. When this occurs, the law is difficult to state one way or the other. In this situation a lawyer can give two kinds of advice - what the law might, on some interpretations, allow and what it definitely allows.