On the previous Air New Zealand flights I did back in November (from BNE) the flights were basically full in both directions. Talking to locals that had been the case ever since they reopened and now with all the marketing, I imagine it's only got busier.
Regardless it's not worth turning up at the airport super early right now, as long as you make the check-in cut off you're fine. There isn't much open at the international terminals and there aren't any queues but obviously, a two way bubble could change that. QF had suggested that the first lounge to reopen in SYD would be the first lounge which would be a nice upgrade for everyone else.
It's also worth noting that you need a travel pass (apply here within 3 days of travel) :
COVID-19 Advice for Norfolk Island | Norfolk Island Regional Council.
Some general comments (things that are harder to find online),
Transport: you need a car, they're cheap to hire but without one it's very very hard to get around. Can't state this enough! despite the island only being 7km wide you can't really walk it, unsparingly there are no taxis although there is one person who calls himself an "uber" although you need to pay cash and ring his mobile to get a ride.
Internet: They've now got a 4G network (started about 2 months ago) but expect expensive data prices and limited data. Where we stayed had a limited amount of wifi per day which seems to be the standard there was free wifi at the airport (probably the only place on the island).
If you're on postpaid your phone will global roam on the local network but no data and calls are $1-2 per minute depending on your AU carrier (Vodafone cheapest then Optus then Telstra). (Scrap that, apparently roaming onto the new network is broken)
Restaurants: The Homestead Restaurant is probably the best (I think it's dinner only and had amazing specials) but you'll need to book this can be done via email, in fact at many places if you don't have a booking you won't get a seat. Also noteworthy, Bounty Bar & Grill (at dinner can get local steak/fish), Hilli Restaurant (good for lunch or dinner).
Wine/Beer: You can buy alcohol at the bond shop, most of the prices are very similar to Australia although most nonperishable items come from NZ so there is a bigger selection of NZ beer/wine in the bottle shop (closed Sundays). There is also a small brewery attached to Castaway accommodation with good cheap local beer but if beers not your thing there is actually a winery on the island although due to a recent drought they didn't have any crops over the last 24 months and so there is no local one, it's imported from NSW.
Food: If you want to self cater you can get most things in the foodland although imported fruit/veg/dairy is very expensive or non-existent. There was always a small supply of local produce and the butcher outside had local meat/milk. The opening hours are also quite restricted 8am-6pm although being from Queensland it's not that unusual.