apologies for the first time post - will provide intro later. the Mining Act is vague however, the definition of search under the act covers what you are after:
Mineral Resources Development Act 1990 3-28/6/97
"search" means search for minerals using no equipment for the purposes of excavation other than non-mechanical hand tools;
so, what the Rangers are interpreting is that by using (searching) with a mechanical device as per your photo on Crown Land, you are not using non-mechanical hand tools. The next question becomes that if you take the dirt off site, this is in contravention of back filling 'as soon as practical'. Who defines practical? They do. Also, there is reference to a 'simple' sluice being OK. Who defines 'simple'? They do.
I would ring the PMAV or DEPI and try and get an answer.