The earlier, London-minted pennies (pre-1919) don't have a mintmark. The "PL" mintmark is only seen on coins from 1951.
1919: Melbourne only (no mintmark).
1941: Melb. (no MM), Perth (either a dot either between the KG designer's initials or a dot after "penny".
1942: Melb. (no MM), Perth (a dot after "penny"), Bombay India (dots before and after "penny", and "I" under head on obverse).
1943: same as 1942.
1944: Melb. (no MM), Perth (a dot after "penny")
1950: Melb. (no MM), Perth (a dot after "penny")
1951: Melb. (no MM), Perth (a dot after "penny"), London (small PL after "penny")
1956: Melb. (no MM), Perth (a dot after "penny")
1961: Perth only (a dot after "penny")