In Singapore, Hong Kong, Shanghai and other places in Eastern and South-eastern Asia, the foreign exchange dealers often place little stamps on the banknotes they handle. The two marks on this note - the H-in-a-circle inside the "0" of "50" and the little R-shaped mark the capital A of "Austrlaia" is pointing at - are both stamps of this kind. The ink which these particular moneychangers have used is obviously soluble in the plastic polymer, so both marks have leached through the plastic to be partly visible the other side.
You can see similar stamps on Australian notes in this thread
, and similar stamps on foreign notes pictured on this website
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