Half sovereigns from the Perth Mint bearing the date 1918 were all struck in later years - 1919 and 1920. This probably explains why Marsh lists Perth half sovereigns of 1919 and 1920 too (not attested anywhere) with reference IDs 535 and 535 and a note saying "not circulated".
The Catalogue of British Commonwealth Coins 1649 - 1971 says that "4 specimens are known" while Krause lists "Estimated 200 - 250 pieces minted."
Not listed in Friedberg. Friedberg's "Gold Coins of the World" 2nd Edition however lists a Sydney half sovereign of 1918, not listed elsewhere.
References to additional information:
[Book] Marsh, Michael A. The Gold Half Sovereign. Cambridge, UK: Cambridge Coins, 1982. pp. 47-51.