I have mostly gone mountain biking in the Port Macquarie area, North Coast, NSW Australia, though I know of a few other good trails. I dont much like mountain biking in Australia because:
1. The trails are not well-maintained: Its a big country and there are alot of fire trails, so being seldom used, they are not maintained.
2. The geology is not favourable: Australia is mostly quartz-based sandstone and granite, which combined with the dry climate, makes MTB less than ideal. Why? Well you dont get deep soil profiles, you get rock ledges and rocky trails rather than smooth paths. For this reason trails are also more likely to have steps. This issue can be resolved by looking for clay-based rocks, whether based on basalts, andesites, feldspathic siltstones, etc. Incertain areas of Australia there are flood basalts. These make particularly good trails if the climate is wet, eg. Cairns, Tasmania because they weather away into clay soils, as opposed to rocky quartz sandstone rubble.
3. The distances between areas are so far: That is really distances between certain rock types and population centres.
4. Unfavourable topography: I actually dont like Australian MTB because of the landforms. The hills are gentle and broad, so you spend alot of time climbing and then quickly downhill. In Japan the differences between flat & mountains are stark, so suits biking. You dont spend alot of time climbing hills.
You are more likely to find a track on the suburban fringes of the cities than in the countryside where there is the most room. These tracks are forged mostly by school kids on weekends. I think these near-city trails make the best riding tracks. The other places to look for tracks are in holiday destinations, where kids take their bikes, and where local kids also maintain the tracks.
So the tracks that I would recommend are:
1. Swamp in Port Macquarie: There is a park-estuary swampland at the end of the main CBD district of Port Macquarie. If you park near the public toilets ot pub, there is a track leading off to the west. Its constructed of wooden planks at first and you will need to climb 5-steps on the way but its a nice short run. A word of caution: The wooded planks get REALLY stippery if it has been raining. Slipping off as I do is not recommended. Basically its a 4km circle, but you can extend the route by crossing back over the main (pub) road and taking the path along the river to the beach and back. You should also watch for people as you tend to pass them at the first time, eg. Turning a corner.
2. Lake Cathie, Port Macquarie: There is an area of state forest just south of Lake Cathie which has a number of forest trails. Driving south through Lake Cathie, upon leaving the town their is a road linking to the Pacific Highway. Take that right turn, and half way along there is a bridge. There are some good areas to park before the bridge, so you can ride around. The tracks on the south side of the road are better.
3. Blue Mountains, NSW: I am told there is some good MTB trails in the Blue Mountains. I am told there is a good track around the old Zig Zag Railway.
4. Fitzroy Falls, Mittagong, NSW: There is a trail here dropping off the plateau of the Sydney Basin into the Kangaroo Valley. The trail runs off to the south of the falls inside Morton National Park.
4. Barrington Tops: Barrington Tops are located between Scone (west) and Gloucester (east) in the Hunter Valley. I recommend the approach from Scone. There is a good place to mountain bike at the top of the mountain passage. You will know when if you look for a cattle grid across the road. It occurs at stark boundary between some scenic alpine pasture and the national park. There are 2 rides in this area. Immediately after the grid there is a loop road going off to the north, and to the south there are fire trails that weave their way down the ranges to the valley floor. I've only done the loop, which was good riding because the loop was basaltic cap over a high point.