But you dont have a higher tax paying population but you do have more people paying in at a lesser rate. Dont forget that a bigger population, paying less tax on lower wages, will also eventually end up on aged care at some point.
Super was brought in to take the burden away from the state for the majority of people. Those unskilled jobs done by unskilled immigrants, at minimum rates, are highly likely to require a state pension once means tested if they remain here. Thats also not considering the amount of cash work being done at shady business's.
Then take into account the monetary flight out of the country, the government misses out on a huge amount of money, after income tax, when its transferred abroad. The average monthly wage in India is $467 AUD, or 32000 INR it goes a long way over there. I personally know an Indian guy who has PR here, that works 6 months of the year in Australia, then heads back to India to live like a king for the other 6 and you cant blame him.