The overpopulation poses at least two problems:
(1) The excessive growth of humanity destroying natural systems across the globe. Five selfish and delinquent countries contribute to nearly 50% of the world's population (notably China, India, USA, Indonesia and Brazil).
(2) The mass migration of humans due to whatever cause (civil war, famine, climate change, natural disasters, sea rise, poverty, recession, the lure of a better life, whatever).
The solutions lie in addressing each of these quite separate driving problems, rather than in addressing for instance 'sea rise' per se. The priorities lie in tackling the biggest and worst problems first - the 'Fat 5' - China, India, USA, Indonesia and Brazil. The Fat 5 are more important than the G20.
The Overpopulation Problem
The high birth rate problem is cultural and is being encouraged and not discouraged by each of the Fat5 governments. It can be resolved by these Fat5 governments each imposing social engineering using financial carrots and sticks. China's one child policy was cruel, but large families living in poverty is also cruel. What about large financial incentives for vasectomies?
The Mass Migration Problem
The second, the drivers of migration are nationally and geographically specific. Two recent examples are the Tamil and Iraqi exodus cause by civil war. These civils wars were human caused and yet the international community (aka the UN) has miserably failed both. The mass exodus was foreseeable and avoidable - neither of course were easy to resolve, but they will happen again until we learn how to avert civil wars, the cause of most mass migration.
Australia's Immigration Problem
Australia's specific immigration concerns are mainly from two causes - refugees which represent less than 2% of the average annual total (but 98% of media interest), and the 98% arriving by plane for economic immigration - people escaping poverty, recession and seeking the lure of a better life in Australia, indeed invited by the Australian Government (but which attract 2% of media interest) .
Solutions become more obvious when the problems are clearly defined.