Just checked a couple of dates for SFO-CHC in December.

For Premium Plus, the fares to CHC are about 600-700 more, or 15-17% more than fares to AKL.

Or for that matter fares on the same dates to SYD or MEL.

So a bit of a premium to fly in directly to CHC. For the fare difference, you can pay for a short NZ flight from AKL to CHC or any other gateway to the South Island so maybe direct to CHC isn't worth it, at east as of now.

Plus since it only died a couple of times a year, potential upgrade inventory would be more limited than routing through AKL?