First day of issue: 9 APR 1979, stamps designed by Max Robinson.
20c Flinders Ranges National Park SA
20c Girraween National Park QLD
20c Mount Field National Park TAS
20c Port Campbell National Park VIC
These are sometimes available on eBay. I might have a few spares if you are looking to complete your set.
Does anyone know, is there a cover with the full set of 7 on it (or even one with just the strip of 5) ?