This is a first prefix banknote. The "prefix" refers to the letters at the beginning of the serial number. In this instance the first prefix was 'AA'.

Obverse (Front)
Dame Nellie Melba - Australian operatic soprano singer. During the First World War, she raised large sums for war charities.

Reverse (Back)
Sir John Monash - Known as one of the best Allied generals of the First World War and the most famous commander in Australian history.