No!
This is a hobby, some do well but MOST find very little.
If the tax man wanted to start taxing finds they would ALSO have to allow deductions for all associated costs ie. ten thousand dollar machines, accommodation, all travel expenses including spiked tyres, Prospecting clothes, licence, GPS systems, epirbs, picks, pointers, safety glasses, backpacks, sunblock, non metallic boots, snake gaiters, knee pads, 4x4 depreciation, caravan purchases, insurances of vehicle and van and the list goes on.
So if someone went prospecting and found $500 worth of gold they would be able to claim tens of thousands in associated costs and never pay tax on their actual jobs. I don't think THEY'RE (ATO) dumb enough to want the tax from that $500 Nugget.
I'm guessing here but there's probably only 1 in a hundred detectorists that make good money some do ok and most bomb and there are many. thousands of cupboards out there with almost and even new detectors sitting in them because the owners watched a gold show (where there's a lot of staging done) and found that the reality of finding gold is pretty tough, that's a lot of tax benefits.