Such a coin does not exist.
The first and only round Australian 50 cent coin issued for circulation was issued in 1966.
There was a round commemorative Proof coin issued in 2006, but not for circulation.
It's just a common half dollar, spend it.
It is worth face value unless it is in a high uncirculated grade it is worth 75 cents.
Australian Archaeological Association was created in 1973.
Round Robin - 1973 is rated/received certificates of: Finland:(Banned)
Royal Australian Corps of Transport was created in 1973.
Caloundra Australian Football Club was created in 1973.
Royal Australian Naval Reserve was created in 1973.
Albert Thompson - Australian politician - died in 1973.
Buster Brown - Australian band - was created in 1973.
Bill Adams - Australian footballer - died in 1973.
1973
Three-cent pieces were minted during the 19th century. If you mean 1873, please see the question "What is the value of an 1873 US 3 cent piece?"