Australia

Not likely, you may be able to slip a NZ coin or something in a stack but of you try to hand a euro note over the staff will say no because their terminals won't know how to handle the currency conversion and will make end of day a pain as they now have a euro note.

You won't get far with that kind of thing in Australia. We have no land borders with other countries so the "natural" blending of currencies with nearby countries at border cities doesn't happen. Nobody has a good idea of what a euro or US dollar looks like, and you'll likely just get told to go get it exchanged for AUD.

Ive never paid for anything in Australia in cash with any other currency except AUD, and i highly doubt any store would legally accept anything else other than AUD (unless a mum and dad store and they are cool with it, but even then probs unlikely)

As far as I’m aware, no, unless stated otherwise in the store. Their value respective to AUD is meaningless. Because then they have to go exchange it to a useable currency, which may incur unknown fees (and is more work, they’d have to pay for). The currency could also be counterfeit, which most cashiers would suspect if someone tried to offer them foreign currency claiming it’s worth multiple times the transaction amount. If you bring foreign currency over a certain amount into the country you must declare it is the only thing I can think of.

You'd need to be dealing with someone who can see the additional value, and has the authority to accept it. For a small, owner-operated shop that deals in mostly cash already, it may work. Otherwise, no chance.