You're right Brian, there is no compulsion to join any industry organisation however if you require assistance from the organisation eg. award information, industrial relations, specific training requirements (annual CPD for licensing), they can be of great assistance. However not paying to be part of an industry organisation does not necessarily save you money, make you more profitable, give you networking opportunities (contacts within builders) etc.
The MBA doesn't just look after builders - they also cover subcontractors - most of their members are subbies. They have regular dinner meetings in local venues eg Roseville Golf club, MBA offices in Glebe etc.
Otherwise BISCOA (Building Industry Specialist Contractor Organisation) is another organisation which may be able to assist (if they still exist).