Carnegie, and probably all the other open age clubs have 'one day' licenses which are $30. Fee structures and race schedules are on the web sites. Vets clubs (over 35) don't have day licenses, but are much cheaper to join for the whole year: about $125. What side of town are you on?

http://www.carnegiecycling.com.au/
http://www.blackburncycling.org.au/index.html
http://www.skcc.com.au/
http://www.hawthorncycling.org/home/default.asp
http://brunswickcyclingclub.com/
http://www.coburgcycling.com.au/
http://www.footscraycc.com.au/

http://www.easternvets.com/
http://www.southernvets.com.au/