Australia

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.

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.

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)

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.

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're allowed to keep it and you're allowed to try and use it to buy stuff but you won't have much luck doing so.

Turning up with foreign cash would be effectively the same as trying to buy stuff with precious metals, jewellery, or rare Pokemon cards - while there's value in the item it's not immediately verifiable, it doesn't fit with the usual workflow of the shop, and they're under no obligation to even consider this as a payment option.

You'd have more luck using foreign cash for person to person sales where bartering and swaps are more of an accepted payment option. Even then expect dubiousness and to lose a lot of the nominal value when it comes to purchasing power.

Maybe some of the souvenir or duty free shops?

But they are already mostly overpriced tourist traps,.and you can bet the exchange rate they offered would be bad.

Why wouldn't you just go to a foreign currency exchange?

Of course you can keep money in other currencies lol. Of course you won’t be able to spend it in stores that only accept AUD.

Those currencies also aren’t “higher value” than the Aussie dollar. They’re just different currencies, that’s what exchange rates are for.