Back from NOU yesterday on QF92 (NOU-SYD). Arrived at airport about 2 1/2 hrs ahead of 12:30 Dep time. Both NZ (flight to AKL) and QF (SYD) check-ins open and processing. So I would assume they opened at least 3 hrs ahead of boarding? Apologies, I didn't specifically ask when do they normally open check-in - if I understand, you're on SB not QF metal?
The hassle even for us arriving slightly earlier than planned is that Immigration/Security check didn't open until 2 hrs prior to departure.
Even though there was an earlier SB flight to SYD (but departed much earlier), Immigration shut up shop in between. So you can't get to the lounge until they re-open - a bit Wellington'esque for those of us who've made the mistake of leaving downtown early expecting to get into the WLG QP etc.
I did notice that there was a couple of flights out to Port Vila in the arvo. So you might be in luck with continuous operation of Immigration/Security for the afternoon "rush".
The SB Lounge is smallish, but a reasonable size for the throughput. Pretty full with both NZ and QF PAX departing 15mins apart. I has some comfy chairs but not a load of food options. But French Champagne of course!