Why is Australian English non-rhotic?

From my understanding, non-rhoticism didn't even start becoming common in the south of England until the second half of the 18th century and early 19th century. Adittionally, from my understanding, rhotic accents were still fairly normal across about half of England in 1950. Given that Australian settlers and convicts came from all across Britain and Ireland, and that most would, therefore, have had rhotic accents, why did our accent end up beinh non-rhotic.

Are there any written records of uniquely Australian accents that were rhotic.