The first requirement for a not for profit organisation to be a legal entity is for that body to undertake incorporation. In Queensland, the responsible State Government body is the Office of Fair Trading (OFT), which is part of the Justice Department.
Many associations rely on the OFT Model Rules for their first attempt at writing a Constitution, as those rules, although generic, do cover all of the issues required by OFT. The Network Committee may be able to supply constitutions from other U3A for comparison.
http://www.fairtrading.qld.gov.au/. You will need to follow the links for Forms then Incorporation.
Incorporation, annual renewal and changes to your constitution each attract a fee to be lodged with the appropriately completed form. Each form will tell you the cost.