When Rowan and I want to do an organised ride, one of the first places we look is on the Audax Australia website:
http://www.audax.org.au
Go to the Events link where you will see links to the larger events, and a link to the Ride Calendar. Look over all the events, or pick the state you're most interested in and narrow your choices to those rides.
http://audax.org.au/public/index.php...d=40&Itemid=97