Congrats on the weight loss! AirNZ has some of the world's most helpful staff, so they should be able to help you on this topic. I'd suggest calling their reservations desk and telling them the situation. First off, they'll be able to identify how full the flight is -- given you're connecting to the AKL-Perth flight, which departs midday, your inbound domestic flight on the A320 (are you departing at say 11am?) shouldn't be as full than if you were on an 8am business-traveler heavy (sorry for the pun!) flight. The reservations agent may be able to either (1) assign you a seat and put what's called a "soft" block on the seat next to you and/or (2) put a comment in your record indicating that you might need for the "airport control" desk to block the seat next to you. Worst case, and assuming that your partner is traveling with you, and likely seated next to you, the two of you would likely be sharing your two seats. I'd be highly surprised, given the photo and your weight, if anyone would even take notice let alone deny boarding to you. I do understand being anxious about the 0.0001% chance, so calling reservations and explaining the situation should allay any concerns.
Have a great flight and do let us know how things turn out.