This set was issued by the Royal Australian Mint Product Code to mark the 50th anniversary of the dodecagonal 50 cents denomination.
The Mint says about it:
"In 2019, the Royal Australian Mint commemorates the 50th anniversary of the familiar twelve-sided fifty cent coin, Australia’s most distinctive decimal coin, which replaced the round 1966 50c piece. This commemorative five coin set showcases each of the five effigies of the Queen that featured on the coins since 1969, and includes a special release gold plated coin to mark its golden anniversary. A significant collectible set, ideal for numismatists."
Three of the coins in the set are available in this only. The all feature the Coat of Arms of Australia on the reverse, and have five different obverses using each of the effigies of Queen Elizabeth II that have been in circulation in Australia on its decimal currency (the portrait by Mary Gillick was on pre-decimal coins only).
The first coin in the set is gold-plated and has the Queen's Second Portrait by Arnold Machin, used in Australia between 1966 and 1984.
The second coin has the Queen's Third Portrait by Raphael David Maklouf, used in Australia between 1985 and 1998.
The third coin has the Queen's Fourth Portrait by Ian Rank Broadley, used in Australia between 1999 and 2019 (this coin was also issued for circulation).
The fourth coin has the Queen's Gottwald portrait by Australian artist Vladimir Gottwald which was only used once for a circulation coin, on the 50 Cents commemorative coin for the 2000 Royal Visit.
The fifth coin has the Queen's Fifth Portrait by Jody Clark, which was introduced in Australia in 2019 (this coin was also issued for circulation).