Local club is HMBA - they race at Awaba once a month. Tracks are always open to be ridden.
Website -
http://www.hmba.asn.au/There are a lot of groups that ride together in places like Glenrock, and also areas less well known. These can vary from social rides to 100-milers.
A mate of mine runs a local blog that is worth checking out. He's an A grader with the mtb club, and a keen trail-builder.
http://pi11wizard.wordpress.com/Some of the HMBA A graders work at Gateshead cycles, it'd be worth going in there.
Hope this helps.