My first thought was roadkill but the marks seem too deep compare to other victims of the same fate either way its been through a lot. I have a lot of trouble finding the norfolk island coins in Adelaide so I would've been devastated if I found that one.
Quote: Ive seen a penny like that before and it was from acid.
American 1 cent coins are plated zinc; acid attacks a pinhole in the copper coating and eats away at the zinc underneath. Australian 20 cent coins are solid cuprnickel, so won't show this surface effect with acid. Acid will attack the entire surface, evenly.
This coin is roadkill.
Don't say "infinitely" when you mean "very"; otherwise, you'll have no word left when you want to talk about something really infinite. - C. S. Lewis