You don't have to worry about that, NZ is (AFAIK/have been flying with them) not known to do op-ups, in fact I would say never. They would rather fly the premium cabins not-full. Their mantra is: if you want to fly in a premium cabin, pay for premium cabin (or One-Ups/use your Recognition Upgrade allocations etc.).

Having said that. It's your honeymoon, I would say pay for PE outright and stretch your budget a little. However, the PE Spaceseat on the 777-300ERs are rather polarizing, it's a "love it or hate it affair". Personally being 5'11" and not exactly small, I do like the Spaceseats and managed to get better sleep on them flying NZ1/2.

You definitely don't want to get stuck in Economy for the 13 hour flight in 10 abreast config, not on your honeymoon anyway.