I must admit I didn't see the 787-10 coming. I supported the A350-900 on the basis that it was closer in size to the 777-200 being replaced, but didn't think the 787-10 would have the range NZ desired. (I did suspect GE engines would've been chosen, though, given the RR-engine grief.)
I've found NZ's own press release, and I see that they've come to some arrangement regarding range:
However, the game changer for us has been that by working closely with Boeing, we've ensured the 787-10 will meet our network needs, including the ability to fly missions similar to our current 777-200 fleet.
I guess that means they have plans on what the 787-10 will do and have assurances that it will be capable of doing so - I assume that means West Coast of the US on decent payload.
Also I see a flexible deal has been negotiated:
In addition to the eight firm orders announced today, the agreement includes options to increase the number of aircraft from eight to up to 20. The airline has also negotiated substitution rights that allow a switch from the larger 787-10 aircraft to smaller 787-9s, or a combination of the two models for future fleet and network flexibility. The delivery schedule can also be delayed or accelerated according to market demand.
I'm interpreting the 12 options as allowing for 77W replacement plus potential growth later on, which suggests an all-787 fleet (of different 787 models) for the future. Perhaps more 787-10 for 77W replacement (LAX/SFO/LHR), and some extra-low-density 787-9 (with increased MTOW) to allow for EWR?