Two things:

1. NZ has virtually stopped issuing award long haul seats in business class. You should check out that forum, but the outlook has not been good even for members of its own program. So you are unlikely to get front cabin seats with them.

2. If you keep your AC award booking, you will be able to request an upgrade to ExecFirst if there are seats open on your flights. However, although the agent told you s/he'g put you on the waitlist, that's not going to help on the day of flight. You need to request at check-in that you want to be accommodated in ExecFirst and ensure they waitlist you at that time. Any earlier waitlist vanishes on the day of flight. This is a tough route to get an at-gate upgrade as there is so much demand for the limited number of seats from cert-using elites as well as J/C revenue customers. Good luck if you keep those bookings.

Did you check out UA to SYD or MEL in business, then NZ into AKL? That's another option is your dates are flexible.

However, smart move to route return via SIN. That's just under the Mininum Mileage since you are going directly down via SYD. Unfortunately you cannot route via Asia both ways or you'd be over the Minimum Mileage limit.