4wd.hk group go to the hills in the NT to drive their 4WD Jeeps, search for their videos on youtube, the track they drive on looks good for MX bikes as well.
I did join up with this group but in the end never actually made to one of their outings. it only cost $200 to be a member and they usually have 2 outings per month. give them a ring and see if you can tag along with them on your bike.