This may be better suited for /r/linguistics, particularly the Q&A thread. But first, it might help to re-frame the question. Australian accents aren't really "British sounding". There are tons of British accents, and Australia has accents that are unique, and actually shares features with the US and Canada but not the UK in some cases. It'd make more sense to ask why Australian English shares certain features in common with the UK, but not Canada.

hi! there's a section in the FAQ on this, although it focuses on American accents ~

• American and British accents

as for Australian & Canadian (and others), here are some more:

• In the year 1900 how easily would an "average" Briton be able to recognise an American, Australian, South African etc accent? Would they know they were from abroad?

• How did American and Australian accents develop into their modern forms from their British origins?

• Why do Oceanian accents sound closer to British than North American ones?

• How did the Australian accent become so differentiated from a typical English accent?

• Is the American and Canadian accent the original British accent?