Unless people in European countries (including Australia and New Zealnd, but strangely not including America - probably due to immigration) start to have lots of babies again, that's unlikely, CP.
Government & independent reports predict an upper bound of 30 million (and from memory the figure for 2100 is also 30 million max, but that's less easy to predict).
Consider that we have less than replacement level Total Fertility Rate (replacement is 2.10 and Australia has plateaued at 1.70 or so - which is the rate used for the average projection).
Ah yes, here it is:
The upper bound projection gives: about 28 million by 2051 and 33 million by 2101 .
The middle projection gives: 26 million by 2051 and 25.5 million by 2101.
The lower bound projection gives: 24 million by 2051 and 22.5 million by 2101.
Immigration is the only thing increasing Australia's population; ignoring immigration rates, Australia would have negative population growth for all three projections (since even the upper bound projection only assumes 1.90 TFR).
Here's a world-map of Total Fertility Rates: Blue is least, red and purple are most. Interestingly, I did not know China had sub-replacement TFR (1.75), though that is good to hear... though it'll be very interesting to see how China handles an ageing population!
http://upload.wikimedia.org/wikipedia/commons/7/78/Fertility_rate_world_map_2.png
Note how Japan has 1.23 TFR. Australia is currently 1.76.
Looks like most immigration will be from Africa and the Middle East. That should be... interesting.