Probably the best thing to do is call the shops that sell race stuff in the areas you wanna ride.
Most of the groups are organized through high-end shops.

What side of town are you on?

there's also this sub-forum:
http://www.cyclingforums.com/f126-au...w-zealand.html (check out the thread about rides leaving from Kennedy's shop) which is a part of this forum: http://www.cyclingforums.com/ This forum also contains the 'Aus Cafe' usenet forum, which takes a couple of days to register for, but that's where most of the Melbournians hang out.