As ironic as it seems in a country with the land area of Australia, but unlimited immigration can lead to over-population. I don't mean that it could lead to the population densities seen in countries like India and China. I mean that in localised areas such as our metropolitan areas. I mean that it could lead to the inability of an area to support that population.
The first thing I think of is water supply. I see the growth of residential areas in the Sydney Metropolitan Area, and I see the possibilities for water supply for those areas. The major storage facility is Warragamba Dam. Our climate is one of somewhat irregular rainfall. I have seen this dam overflow, and I have also seen its water level drop a great deal. Already the height of the dam wall has been raised above its original design height. There are no other sites for dams because any dam would hold back water flow into Warragamba.
The water supply problem exists all along the east coast and the inland. The 48 States of the USA, which cover the same area as Australia, can support their populations because the land has abundant water throughout. Similarly for Europe. This continent has been inhabited for 50,000+ years. Why wouldn't it have produced a similar population size to other places? Simply beause the lack of water meant that the inhabitants did not have any time after attending to mere survival to develop the civilisations the developed elsewhere.
It immigration continues unabated, Australia will not be able to provide the essential need for survival - water. If that happens, we have over-population.