Bear in mind using metal/gold detectors is a big no no in regards to fossicking. To use a metal/gold detector than you you need to do it for prospecting purposes only, fossicking for rocks and minerals other than gold and diamonds.
Basically it's just how things are worded in WA legislation & doesn't mean too much in the scheme of things.
You still need a WA Miners Right to either Fossick or Prospect in WA.
If you're Prospecting you can use a metal detector & search for all minerals including gold & diamonds.
If you aren't using a metal detector & aren't searching for gold or diamonds you are Fossicking.
It's splitting hairs really because anyone with a WA Miners Right would/should just say they are prospecting to cover all bases anyway?