I don't know why Victoria is considered shallow? If you mean by digging alluvial shafts, then yes places were shallow - if you mean to detectors - then shallow is a very subjective term. A quick study of the list of the biggest nuggets found shows very few were shallow and most were several feet down and out of the range of any commercial gear.
There are a few blokes trying a few new things around the triangle with detectors and various types of set-ups, and a lot of bigger and better hand-made coils are starting to be made and used - a few small experienced prospecting groups with electronics/fibreglassing skills are coming together to design and make gear with some good results both in testing and in the field at far deeper levels than seen before. There are of course the very big commercial coils getting about as well.
As technology has and is changing, what were once considered 'cleaned -out' areas are now producing the goods again. Its why a lot of us will not give up details of cleaned out old patch sites because its simply a matter of waiting for the technology to improve. We have seen the progression from - VLF to SD to GP to GPX and all were a marked improvement.
The bad news is that things are only going to get more expensive and to get any depth, more complex and heavier I would imagine.
As for people not filling in holes - well, the list of don'ts I see all the time is endless - rubbish, cigarette butts, driving off tracks, fires on fireban days, digging into creek banks, dropping turds at random, picking wild flowers, taking firewood, etc etc.
