You may have rights under the Trade Practices Act in relation to say, defective goods, or misleading or deceptive conduct. The contact for issues in relation to the Trade Practices Act is the ACCC. More information and links on our site here.
If your complaint is more to do with incomplete building work (e.g. by a plumber)
you should contact the specialist building regulator in your state.
You'll find that contact information on our site here