We really enjoyed fossicking near the northern Queensland town of Forsayth.  The caravan park owners there were lovely people and very helpful.  One other couple that was camped near us found a large gold nugget - but they'd been searching for quite a while and had a very sophisticated metal dectector (no - that's not sour grapes on my part!).  Also in that area is Agate Creek which has got some lovely agates scattered around in a dry creek bed and O'Brien's Creek and Mount Surprise both of which are good bases to look for topaz.