The Perth Mint’s annual Australia Sovereign (also called the Australian Sovereign) is a prestigious 22-carat gold coin featuring a design inspired by historic sovereigns made at the Sydney Mint between 1855 and 1866. As the first overseas branch of Britain’s Royal Mint, the Sydney Mint was opened in 1855 to make sovereign coins from Australian gold. For the first 11 years they were distinguished from London versions by a specially prepared "colonial" design. A testament to the skill of Australia’s early minters, these highly successful issues continue to attract significant interest among modern collectors.
The regular coin has the same specifications as the British gold sovereign, which is a continuation of the Imperial (pre-decimal) sovereign - produced, among others, by the Perth Mint between 1899 and 1931 when it was a branch of the Royal Mint.
Since 2018, the Perth Mint also issues a Double Sovereign, which has the same specifications as the British Double Sovereign; its gold content is slightly lower than a half ounce of gold (0.4712 AGW), with 22 carat purity.
Unlike the first two "double sovereign" issues, this coin is called the Australian Sovereign 2021 Gold Proof High Relief Piedfort Coin and is considered by the mint a "piedfort" of the regular sovereign of the same year - that is, it has the same design and diameter but is double the thickness. However, it is denominated as 50 dollars and not 25 as the "single" sovereign.
The Mint says about it:
Twice the weight and twice the thickness of the traditional sovereign, this special high relief "piedfort" version also incorporates an extremely significant privy mark; it represents the age of Her Majesty Queen Elizabeth II on her recent birthday - a remarkable achievement by Britain’s longest-lived monarch.
This Sovereign has been struck with an extra-deep blank to showcase its superbly detailed design in high relief. Additionally, with its extra-thick planchet, this coin is reminiscent of traditional "piedforts" (pronounced pee-ay-fore), which take their name from the French words for "heavy weight".