You can of course contribute privately to this balance as well, and in fact in some cases the government has matched contributions.
Super is not taxed*, though you can't withdraw till you're over 60.
It does have a lot of flexibility as well. It's possible to set up a 'Self Managed Super Fund' which can be used to buy property, or invest in other ways (though this is fairly tightly regulated).
This is of course not connected to the availability of a pension, should that money ever run out. Which makes it all feel pretty far left.
The difference between the U.S. and Australian systems is that you own an account from the mandatory contribution, while an American owns nothing, it's just a tax to fund current and future benefits that are at the whim of current and future governments.
It's often hard to map political positions across nations. Most recently, George W. Bush was a big proponent of moving toward an "ownership society" with a Social Security system that looked more like Australia's and it was our left that fought the idea.
Australia's system is terrific, performing well and should be a model for the U.S.
To answer your question: That 2% in an optional 401(k) tax-advantaged private account is above and beyond the 12.4% that goes to Social Security. How much of that 12.4% the employee will ever see depends on his lifespan and the continued benevolence and fiscal prudence of the U.S. Congress.