Tony Jones opened by saying that Mark Latham had belled the cat by pointing out that you can't propose to limit population unless you can limit immigration. So why wouldn't the government promise to limit immigration? Jones to his credit kept the debate on population and immigration rather than sliding across onto asylum-seekers in the way for which Q&A is notorious. (A common pattern with Q&A is that, whatever the nominated topic, before long the conversation gets on to asylum-seekers and tends to stick there.)
Scott Morrison argued correctly that to follow the Third Inter-generational Report scenario towards 36 million Australians in 2050 we would need to get net migration down by nearly half, to 180,000 or less by 2012. If the government was pretending that it wanted to reach a figure lower than 36 million, then it must get net migration well below 180,000. Would it commit to doing so.
Of course Tony Burke wouldn't commit to doing any such thing. He talked nonsense about how it wasn't a matter of how many people you had in Australia but where you put them. Morrison replied that only 10% of immigrants settle outside the cities, so it's dishonesty to pretend that Burke's government could re-ruralise the population. (Not that this would solve problems like Peak Oil). Burke came across as a skilled spin-doctor, but with no solutions to offer. Morrison was far more impressive. 90% of what he said was factually true. But he too went to water as to whether the Coalition would in fact reduce net migration to well below 180,000 a year, as he seemed to be advocating. I had the impression he was constrained by lack of party support for the direction that he personally might see as necessary. (The Coalition's policy, as stated at http://www.liberal.org.au/Latest-News/2010/04/29/Coalition-rejects-PMs-population-target.aspx , is almost as vague as Labor's, and so far from promising to cap population it talks of "a sustainable population growth path". The Greens policy is in some ways worse.)
And there the debate petered out, with Burke stonewalling like Slasher Mackay.
What was positive was the firm focus on population, and the recognition, at least by Jones and Morrison, that immigration numbers are a crucial lever for controlling population. As Morrison remarked, we need to concentrate on things we can actually control.