Basically, NZ paid UA for taking you NS rather than the extra leg on them. NZ would owe you mileage credits, not UA. They needed to "free up" seats and UA had one to your destination. UA didn't have cake and eat it too- NZ did.
My humble opinion. If you believe UA owes you miles, then you should call and get them. The miles you spent on UA would have been the same had you booked your trip the way you wound up traveling, right? NZ was apparently oversold. If you are owed miles, it's from NZ.