Get a minimum of three bids. Estimates for an average-sized job should be within a few hundred dollars. Be suspicious of anything that is substantially lower or double the price of the rest, and watch out for hidden fees, like charges for travel expenses. They could be signs of a home improvement scam. A good plumber will not nickel and dime you like this, and many of us will offer free estimates.
Plumbers in Australia have licensing requirements that differ from state to state but it is generally accepted a 4-year apprenticeship with a further minimum experience of 2 years (6 years total) and a further curricular requirement as a benchmark for licensing. Licensed plumbers are also expected to maintain minimum relevant training requirements to maintain their plumbing license