Model Rules

The basic form of the constitution of an incorporated not-for-profit organisation is generally drawn from the Model Rules attached as regulations to the Associations Incorporation Act in each state. Organisations may modify this basic format, but some elements are compulsory; in particular, if you do not make provision for certain things the law will read them into your constitution anyway - "Where in relation to any matter the model rules make provision but the rules of an incorporated association do not make provision, the provision of the model rules shall, in relation to that matter, be deemed to be included in the rules of the incorporated association. "

If you're starting up a new organisation it's a good idea to make a few small changes to the model rules.

1. Put in your objectives. Technically, you don't have to include your objectives or goals or mission in your constitution because you've already done that in the statement of objectives you lodged with the department when you were getting incorporated ("The objects of an incorporated association are the objects stated in the statement of objects that accompanied the application for its incorporation under this Act"). However, including them in the constitution as well makes it a lot easier for people to understand what you're on about.
2. Put in criteria for membership. The model rules say that every new member has to be approved by the board (many small groups ignore or overlook this provision, but it's there). It doesn't, however, say what the board should take into account in making these decisions, and this can place the board in a difficult position in the very rare circumstance that they do actually want to keep somebody out. If you put in something unspecific like "Committed to the objectives of the organisation" this gives you some cover.
3. Put in (if you want to) provision for voting by phone or email. The model rules don't cover the electronic age, and it's not at all clear what they allow and forbid. If you want to be absolutely certain, write in your preferred options.

That aside, you can make any amendments that aren't ruled out by the words of the Act, but most groups don't bother.