When we are talking about sustainability and use of resources and land, etc, we unfortunately have to talk about a red herring called "birth control" – or "birth rates" to be more political correct.
We cannot expect the world population to grow infinitely in a limited world with limited resources. More land for more people results in less land to grow food or to maintain a working eco system.
The birth rate in Germany 2006 was about 0,016 ( http://www.destatis.de/jetspeed/portal/cms/Sites/destatis/Internet/EN/press/pr/2007/06/PE07__228__126.psml )
meaning that every woman had 0,016 children is her life. Not very much.
New Zealand was about 4.2 in 1962, now it is about 2( http://www.stats.govt.nz/~/media/Statistics/Methods%20and%20Services/Tables/Births/fertility-rates-maori-and-total-mothers.ashx ).
That means that Germany’s population is decreasing while NZ’s keeping constant. This does not include new immigrants coming into NZ, while people are rather leaving Germany.
To explain this situation with my own experience: It is very expensive to have children in Germany and many people are focussed on their career rather than on family. New Zealand is still a family focussed country, but it is increasingly costly to have kids.
To keep it short: Obviously, a birth rate of 2.0 is a sustainable number where world population is kept constant. That should be our target. Otherwise there is no need to talk about sustainability, because how ever sustainable we will be, it won’t work long term. This is just a mathematical and quite logical fact.