Just scrolling through Adams posts Im guessing Vic probably.
With the exception of the areas in NP where you can prospect (eg Chiltern-Mt Pilot & Bendigo NP as 2 examples) I would say absolutely a no go.
If in doubt contact Parks Victoria but I can assure you metal detecting outside designated areas in a NP will put you in hot water.
Firstly, theres the relic laws, meaning even pre decimals shouldnt be removed from anywhere in Vic, then there is the historic overlays, and finally NP are pretty much made to protect the flora inside their boundries.
So even outside of NP relics are meant to be left as they are, at least thats what the law is for.
Dont have info for other states.