Once you have the proof to validate that it is a defect, you would need to talk to the builder. You would need to enforce your rights and agree on the solution to solve the defect. This is where the warranties and insurance can come in handy.
If the builder agrees to fix the defect, then you just need to make sure that they will do it. Else, you can file your issue as a building dispute. A specialist lawyer can help you with the legal process to do it.
They can help you with the legal processes of Fair Trading and NCAT. Since these tribunals are in charge of handling building disputes, you can count on a specialist construction lawyer. They can guide you on the step-by-step process of enforcing your rights as a homeowner.