Demographers at Waikato University's Population Studies Centre say that unless New Zealanders start breeding more, New Zealand's fertility rate will slip below replacement level. And? I don't actually see why this would be a Bad Thing. Or why a growing population would be a Good Thing. Or indeed, why this should be any concern of government (or indeed anyone) at all.
Our fertility rate is an aggregate of people's individual reproductive choices. And those choices are fundamentally personal and the sole domain of the individuals concerned. It is no business of government how many kids I or anyone else has. It is no business of government whether any of us breed or not. Government simply has no legitimate interest in what goes on in our bedrooms, or in whether those activities result in children or not. It is simply None of Their Fucking Business.
That can't be right. It would clearly be a bad thing if the entire human race died out, for example. It's logically possible that the aggregate impact of individually permissible personal choices would have disastrous consequences for the world at large. If so, we have a legitimate interest in taking collective action to avert disaster or create a better world, and that's what politics is for. We all might reasonably agree to structure the general institutions of society so as to incentivize socially beneficial choices, e.g. to replenish the dwindling ranks of humanity. This can't plausibly be denied in principle.
In practice, of course, there are plenty of grounds for objection. For one, world population does not appear to be excessively low. So if some nations want to increase their local population, they should allow more immigration. (Cf. adoption.) And I assume NRT is worried about setting a bad precedent or 'slippery slope' for social conservatives to start meddling in personal lives in a more intrusive way. My point is just that as a matter of principle, our liberal individualism should not be so extreme as NRT's above. It's not really true that state interest in the individual sphere is absolutely illegitimate. Despite NRT's fears, we can draw a principled distinction between, say, saving the human race and persecuting gays. (Really.) The hard work is discerning the precise borderline. The absolutist can avoid this hard work, but only at the cost of being, well, you know, wrong.