I have an original AU50 1866 Sydney Mint Sovereign, on which the design of this coin was based. Yellow gold alloy, 22ct gold 2ct silver.
The alloy for Australian gold coins changed to 22ct gold, 2ct copper, (consistent with the rest of the British gold coins), in 1868.
The first and second dates of the original coins (1855 and 1866), carried an obverse design of Victoria, which was similar, but not the same, as Young Head British sovereigns. The reverse design was very distinctively different, as seen here.
The later Sydney Mint obverse design (1857 thru 1870), was very similar, (but not the same), as the Victoria obverses found on the coins of Newfoundland. On the Australian coins, Victoria had a garland of Australian native Banksia on her head.
That, and the gold / silver alloy, is what motivated me to acquire the 1866 coin, as mentioned above.