The colony of New South Wales had two periods when its own distinct coinage was minted.
The second period was between 1855 and 1870. The Sydney Mint was opened as the first branch of the Royal Mint on 14 May 1855 and the first coins were struck on 23rd of June that year. The Sydney Mint produced gold sovereigns and half sovereigns from dies supplied by the Royal Mint, and engraved by James Wyon. The British Government required that these coins be different to those minted in England. The obverse was soon "localised" though, with a sprig of banksia being added to the fillet in Victoria's portrait. Confusingly from our modern point of view, the coins had AUSTRALIA promptly displayed on the reverse, even though Australia only came into being as a country in 1901.
Originally these coins were regarded as legal tender in New South Wales but in 1857 the British Treasury amended the ordinance to include "all the colonies of Australasia". This served to annoy both Melbourne and Adelaide which were petitoning for thier own mints, they reacted by spreading rumours that the Sydney coins were inferior to their British conterparts. A "snap" inspection by the home authorities revealed that since the coins where alloyed with silver as well as copper , they were in fact actually worth even more. By this time they were already circulating in Newfoundland and India and even unofficially in England itself. In Febuary 1866, the Sydney Mint Sovereigns and Half-Sovereigns were declared legal tender throughout the United Kingdom by Royal Proclamation.
In 1870 it was decided that there should only be one type of sovereign. The Australian obverse and reverse were abolished and the Sydney Mint (as well as the subsequently opened Melbourne Mint and Perth Mint) only struck Imperial-design sovereigns thereafter.