For pennies dated after 1938:
London, England - "PL" mint-mark
Bombay, India - "I" mint-mark
Perth Mint - "." between the "K" and the "G"
Perth Mint - "." after the letter "Y" of the word "PENNY"
Perth Mint - "." after the last "A" of "AUSTRALIA"
Melbourne - no mint-mark
So look for either a "PL", "I", "K.G", "Y." or an "A." mint-mark!