A warranty is a promise that work or a product will perform for a period, and a statement of what happens if it does not. On a trade job, three layers sit on top of each other. The first is the law. Under the Australian Consumer Law, services to consumers come with automatic guarantees that they will be done with due care and skill, be fit for any purpose the customer made known, and be finished within a reasonable time, and goods come with guarantees of acceptable quality. These apply whatever your paperwork says and cannot be signed away. Residential building work also carries statutory warranties under state home building laws, with periods set by each state. The second layer is the manufacturer's warranty on the products you install. The third is your own warranty on workmanship, which you choose and write yourself.
Your written warranty adds to the legal guarantees; it does not replace them. If you give consumers a written warranty against defects, the Australian Consumer Law sets out wording it must include, which the ACCC publishes.
In practice
A plumber installs a hot water unit with a 12-month workmanship warranty, and the manufacturer covers the cylinder for five years. In year three the unit leaks from the cylinder. That falls under the manufacturer's warranty, and the plumber helps the customer lodge the claim. If instead a joint the plumber made had failed in year three, the 12-month workmanship warranty would have expired, but the consumer guarantees might still apply if the joint was not made with due care and skill. The plumber's written period does not decide that question on its own.
Write warranties in plain terms on the quote or invoice: what is covered, for how long, what is not covered, such as damage from misuse or work by others, and how to make a claim. A clear warranty is a selling point. A vague one invites the customer to read it as broadly as they like.
For what the consumer guarantees require in a particular case, check with the ACCC or your state's consumer affairs or fair trading body, and for statutory warranties on building work, with your state's building regulator.