The reverse features the Ensigns Armorial of the Commonwealth of Australia, as authorised by Royal Warrant 7th May 1908.
At centre, simple shield enclosing a cross of Saint George on which are five six-pointed stars, around the outside of which are six small escutcheons, i.e., small shields. The shield is supported by a kangaroo and an emu standing on a grassy mound. Above the shield a crest, the seven-pointed star of Federation on a wreath and on a ribbon at the base the motto ADVANCE AUSTRALIA is inscribed.
Around above, the denomination SIXPENCE; below, between arrow heads, the date 1921.
The mints did not use mint marks, so coins struck by the Melbourne Mint and the Sydney Mint are indistinguishable.