Presume your HNL-SFO segment is on UA, so you are permitted to book A class to get UA's domestic First cabin if D/Business is not an offered class on the flight.
Also presume your SYD-HNL segment is on AC. Get your agent to waitlist in C class and let AC's yield management release the seat if they can. If they don't know you're waitlisted, they can't very well release a seat!
The use of "Y" as a booking code is a worry, unless you've made it a Y to show you are in the Economy cabin. Ensure your agent has the NZ/UA segments in M class, and the AC one in H.
Cheers.