I would pay a Business class fare for a top quality seat (BA, Qantas, Emirates) or close (Cathay, Singapore). The nightmare is if you book on a middling airline (eg Thai) and get an old plane then business class if woeful. It's ok on the newly refurbished planes
Booking in advance makes a big difference to prices - business travellers tend to book late and pay much more as a result.
Think about premium economy and spend the savings on a five star hotel for the night and a blow out meal with fine wines (or whatever luxury you want - 3 days exclusive fly fishing?). You will remember it longer than getting to NZ, which is a long way whatever class you are in.
It is rarely worth paying your own money for First, unless you have so much as to not notice it. Having said that, I am still delighted to get an upgrade or have the firm pay when business is full.