You can generally get U seats if you are prepared to go via AKL-SYD/MEL and can book 3 months or more in advance. I just made some MEL-AKL-MEL bookings for may and was able to upgrade from N to J both ways. Saturdays seems to be best and the maximum this far out for any flight seems to be U2.

The best way I have found is to use the CX ITN and look for U seats.

eg: Here is a sample URL that shows some U availability in May (at least today (25th jan) it does )