As above, you’ll need to have to your AirPoints # in the booking to use your Koru benefits when flying NZ. Therefore I very much doubt if you use your UA # that you’ll be allowed to use your Koru benefits.

AirNZ insists that passengers can’t ‘double dip’ and accrue mile to UA whilst also using holding your NZ number for ‘status’ benefits. The T&Cs state:

”In the event that Airpoints Dollars and or Status Points have been accrued to your Account for travel that you also used to gain frequent flyer or loyalty benefits under another Airline's loyalty or frequent flyer programme, we will remove the relevant Airpoints Dollars and Status Points from your Account.”

Before getting too focussed on earning to UA, check if the NZ flights (and flights to/from Australia) actually earn anything useful. If not then just use your NZ ff#.

Not recommended, but it could be possible to first use your NZ number in the booking for seating, baggage and lounge benefits, and then ask gate agents to change to your UA prior to boarding.

However, when doing just this myself (once only 12 years ago), I was explicitly asked by AirNZ staff if I’d earlier visited the Lounge, and the implied consequence was that if I had then they would not change the ff#. I pleaded ‘ignorance’ and the change was made - but certainly not worth the hassle (at least for domestic flights). Might be a possible option for the long haul.

Note also that if you manage to do this, then it is likely that all subsequent sectors will also be changed to your UA #, and you’d have to carefully check which ff# is showing for later sectors and change.