Can you please help me identify this coin? Or what could've happened to it?

So I have this coin, that is not like others… and I can’t seem to figure out what its deal is?
This is an Australian coin by the way, I hope you guys can still help!
Here is the picture of it, plus a comparison to a ‘regular’ Australian $1 coin. It’s quite mutilated on one side which is the most perplexing thing about the coin, that and it’s much bigger than a typical $1 coin but the same thickness (so it hasn’t been flattened).
So… if anything can tell me a bit about the coin? I can’t seem to find anything…
Thank you!