There isn't much in the way of technical trails within riding distance (the only one I know of would be the Yarra trails). However, if you are willing to drive for a bit, there are many places close-ish to Melb that have great mountain biking. The main one would be Lysterfield (great set of tracks plus the Comm Games track). From there we get further out with: the You Yangs, the Dandenongs (not a heap of technical stuff - but it has a great hill!), Whittlesea (the scout camp - not sure if it's open to the public), Officer (Anderson Scout Camp, also not sure if open to the public). These are the only ones I know of around Melb. If you are willing, there are some great mountain biking places around Vic, such as: Beechworth (great mtb park), Mt Beuty (Hosting the XC National Champs) (Both of these are on the 2006 GVBR - can't wait!!
), Castlemaine (sp?), Ballan, Ballarat, Geelong, Mt Buller (has its lifts open for DHing), and Eildon (Hosted last year's National Champs). All of those places listed (except the Dandenongs) host state-level cross country races, so you can expect the courses to be fairly hard (pretty well all of these places also have multiple tracks).
I have raced at the You Yangs and Ballan (with Whittlesea soon to be added to that list), and they are both great tracks with some great technical sections.