It's raining women - or is it?

There has been talk about a surplus of women, especially in the 30-34 age group.

Demographer, Paul Callister has been researching this;
Callister said it was difficult to pinpoint exact reasons for the surplus of women but said men – particularly young men, Maori and Pacific Islanders – failing to fill in census or statistical forms was one major reason.

So I interpret this as meaning that the surplus on paper doesn't necessarily equate to a surplus in practice. (Perhaps some of these are the same "young men" that don't fill in birth certificates either.)