The Housing Industry Association (HIA) and the Masters Builders of Australia (MBA) are the main associations for builders.
They will take complaints about members, but their primary obligation is to the member.

You can contact your Consumer Affairs / Fair Trading offices.
Some states have specialist building regulators and tribunals with dispute resolution processes that you can use.

See State information below.

QUEENSLAND
Queensland Building and Construction Commission
Phone: 1300 272 272 (or 07 3225 2800)
Website: click here

NEW SOUTH WALES (for Southern NSW see also ACT)
Department of Fair Trading
Phone: 13 32 20
Website: click here

ACT
The Office of Regulatory Services
Email: ors@act.gov.au
Phone: (02) 6207 3000
Website: click here

VICTORIA
Building Advice and Conciliation Victoria (BACV)
This joint Building Commission/Consumer Affairs Victoria service helps consumers and builders resolve and prevent home building disputes.
Phone: 1300 557 559
Website: click here

TASMANIA
Department of Justice
Building Standards and Occupational Licensing
Phone: 1300 366 322 (within Tasmania) | +61 3 6166 4600 (outside Tasmania)
Website: click here

NORTHERN TERRITORY
Office of Consumer Affairs
Phone: (08) 8999 1999 or 1800 019 319
SMS: 0401 116 801
Email: consumer@nt.gov.au
Website: click here

SOUTH AUSTRALIA
Office of Consumer & Business Affairs
Consumer and Business Services (CBS)
Phone: 131 882 (ext 5)
Website: click here

WESTERN AUSTRALIA
Building Commission
Phone: 1300 489 099
Email: BCinfo@commerce.wa.gov.au
Website: click here