I relate it to the amount of "day" and "night" spent aloft.
QF30 is currently scheduled:
- 12:30 Depart LHR
- 08:20+1 Arrive HKG - 11:50 flight time
- 09:50+1 Depart HKG
- 21:40+1 Arrive MEL - 8:50 flight time
Assume equinox with notional Sunrise/Sunset at 6am/6pm. On a pro-rata basis, the following timings are pertinent:
- From LHR-HKG:
- The sun "sets" ~3:15 after LHR departure,
- "Night" lasts ~7:10 &
- The Sun "rises" ~1:25 before HKG arrival.
- From HKG-MEL:
- The aircraft takes off in daylight,
- It travels during the "Day" for ~6:05 &
- The Sun "sets" ~2:45 before MEL arrival.
To me the former segment is a "Night" flight, the latter is a "Day" flight.