Been a while since I looked at this, but if I recall correctly that the correct answer is "it can't" (as 2D runs largely over VF's network, which is somewhat different to Telecom's).
But.... it does "sort of" work in reality, specifically it works in the main centers but not out the country and small towns.
Is this correct what I recall?
Because if so I'm going to get 2Degree's Huawei IDEOS U8150 and pop my XT SIM card in it, as I spend 99% of my time in the Auckland CBD and won't be bothered if it doesn't work anywhere else.