Your other thread was closed, since double-posting is an FT no-no, but here's the deal.

Since NZ and UA don't have a JV, UA and NZ must price on their own - they are still competitors. Obviously, they are not competing price-wise on this route, at least UA isn't on the NZ nonstop. After looking this up, it turns out that for whatever reason, the UA site only allows you to book into full fare economy on the NZ legs, which sounds like its the reason for the price jump. Whether UA isn't selling any seats in the lower fare classes, or just thinks that they can get it, it is what it is.

NZ indeed has the lower fare, but given you're looking for PQD, presumably you're also looking for PQM/PQS. Doing a search, its notable that the $1446 fare on NZ, books into L class and that won't help you there either - L class on NZ doesn't earn mileage at all. Would it have made you feel better to book the cheaper ticket, and then find out you don't earn mileage at all?

Now, I don't know the details of NZ booking, but looks like moving into an Economy Flexible booking, which would earn mileage, brings the price up to $2600. The UA routing through SYD takes longer, but is coming up at $1979. So $500 more than the NZ fare, for one that earns full PQM/PQS/PQD as far as SYD. Obviously, the choice is up to you on which works for you between time, budget, and mileage earning (among possible other factors), but those seem to be your options.

As said elsewhere, *A is a marketing alliance, not a merger of the airlines involved. And it has been noted in dozens of other threads on FT that pricing can vary by airlines marketing the very same flight. Sometimes, the marketing carrier prices lower, sometimes the operating carrier does.