I forget the original reasons, it was some time back. There is a statement here: http://www.treasury.govt.nz/budget/2010/is/04.htm

I seem to remember it was partly prompted by a new government wanting to prove they were fiscally responsible, but also I think there has always been quite an awareness as there is a huge earthquake reconstruction fund (now entirely in external assets) in case of disaster, which would prompt some thinking about assets and liabilities.