I think that the "legitimacy of the US order" refers to the evidence that will be presented to the US grand jury, not the search warrant in New Zealand. The Mutual Assistance in Criminal Matters Act [1] requires that the Attorney General be satisfied that the request be related to a foreign criminal matter, but they do not have to ensure that the order would be held up in the US.
That makes more sense, though it still looks like a dangerous abridgment of due process if the country that actually seizes the assets feels no need to launch an independent investigation to find out whether or not the allegations have any merit.
I hope the NZ government revises their extradition treaty in this light. USA has a legal reputation of trumped up charges to force a plea deal despite no trial. Especially if they can play the card of 'money laundering' which legally allows them to seize every asset and cash.
We are now between two stable states - the US will not be as an important player in the early 21st century as it was in the late 20th - and a lot is at stake right now. I can only hope we can all elect governments wise enough to ensure a smooth transition because trying to prevent - or even postpone - it will only make it more violent and unpredictable.
[1] http://www.legislation.govt.nz/act/public/1992/0086/latest/D...