Originally Posted by nzkarit

One word: Reserves.

The plane has to land X fuel on board. When it commits to SYD, it is actually committing to holding at Sydney, having a go at landing, diverting to MEL, holding, having a couple of attempts at landing and still not using final fuel reserves. There is a reason don't hear about airliners running out of fuel.

Landing at primary without alternative reserves or alternative with final reserves results in a bunch of paperwork with the regulator. May results in pulling ETOPS or similar certifications for the airline.

Additionally there is the the ETOPS issue. When the plane is at it furthest points from an airport there needs to be enough fuel to be running on one engine, at 10k feet (cover situation of depressionrisation at the same time as an engine failure). And there is nothing between NAN and AKL. Suspect on the JFK-SYD would more likely see AKL or NAN diverts as will fail the ETOPS requirements.

SYD-LHR ETOPS at the end of the flight when lower on fuel will be less of an issue as the airport density is higher, so ETOPS fuel requirements won't be such a factor.

For the first PR Sunrise flights they will likely not sell a bunch of economy seats and not take cargo so they can be sure no issues.