Hope I'm not too late on this, head for Minyon Falls. Some awesome dirt to ride on in that area, and some stunning scenery. A lot of it is on old fire roads. If you're really feeling adventurous, you can ride a loop from Mullumbimby to Wilson's Creek, Huonbrook, Red Rock road (or something similar) down to the falls, then returning to Mullum through Coopers Creek and Repentence Creek. Further north, you can quite legally ride on the old fire roads in Mooball National Park, that's an area that I'm keen to explore a little more myself.
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I am clinically insane. I am proud of it.
That is all.