Aussie here!
They are just called $xx International Stamps. They'd be between $3.50 - $4 depending on destination.
You'll be able to find them on Australia Post Websites, although not sure if they ship overseas.
Let me know if you need some and maybe we can work out a stamp swap

Very similar to International Forever Stamps the US ones, but does anyone know the name of the australia ones for self addressed stamps as I only wanted to send a letter c5 size? Any help would be appreciated