Last I read was 25% of Aussie citizens were post-WW2 migrants and their descendants. Don't remember the source.
I suspect it is an an art of inexact science to analyse census results about ancestry. Many refuse to answer what their major ancestry was, and many may answer he has English, Irish, and German descent and it could constitute three counts in the census results.
Having said this, non-British/Irish descent Australians with European ancestries constitute 20's percents of Australian population sounds right.