There will be a lot of climate disruption in Australia no doubt, (already happening with fires and floods and rising seawater temperatures) but at least a lot of land won’t be under water or destroyed by high salinity like the Ganges-Brahmaputra-Meghna Delta.

2015 article from a quick google search….

https://blogs.worldbank.org/endpovertyinsouthasia/bangladesh-challenges-living-delta-country

The biggest natural defence Australia has is its hostile deserts to the north west and centre but when 1 or 2 or 30 million people decide they have to move to survive that won’t stop them.