Not sure of it's actual origination,however in Afrikaans the word 'aap' means "ape" and an "aapie" is a "small ape" or "monkey"
So although Steve W's version is plausible I expect this is the real reason for the name so enjoyed by the Aussie's/Kiwi's.
By the same token there's no end of 'sheep' jokes over here either
It all evens out on the Rugby field..5555