You’re experiencing essentially the same issue as was described in https://bugs.kde.org/show_bug.cgi?id=516550. “Australian English” isn’t truly a different language from English, which means you don’t have an opportunity to select Australia as your region for locale purposes.
We’ll have to get this fixed.
In the meantime, you can set LANG=en_AU.UTF8 in ~/.config/plasma-localerc.