That's not true, at least not in Victoria and many other states I'm aware of. Again, I know because I used to install/configure cameras, including in carparks and even a large shopping center. It's also why most cameras don't even include a mic at all

Feel free to show us the exception in the law.

Furthermore, it wouldn't make any sense as it would be the world's most obvious loophole and violate people's privacy. As literally everyone could just claim they were trying to identify theft. And, it would still allow people to listen to the conversation if they were watching the nvr without being a part of the convo

In some states, you need to be a part of the conversation. In others, you need permission from the other side too.

This isn't just Australia, world-wide in most countries a sign doesn't count for consent

There's a reason you never see security footage on TV with audio recorded. You can possibly record a crime being performed as evidence, but, supermarket cameras would be recording all audio, including unrelated audio ..