A plumber is a trades person who can fix water and sewage piping problems you may be having. A plumber can also do new pipe installations and toilet connections.
Yes, when you are looking to employ a plumber, you should find one that is licensed to operate in your state. Each state in Australia has its own licensing and registration scheme. You can ask a plumber for their licence number and it should be listed on your states registry.
If you are building a new home a plumber can provide advice on where pipe locations need to be built in advance which makes it easier when it comes to pouring slabs of concrete.
Gasfitting is pipework involved in getting the supply of gas from the street to your stovetop and hotwater system. Plumbers have to be licenced to perform gas repair or maintenance.