airport open more hours doesn't mean more flights, it may just mean flights are more spread out.
For flights to take off in evening means they must fly in at that time as well (no airline wants aircraft to sit on ground for more than ~30 mins, especially at QN where they might get stuck if weather comes in).
Who wants to arrive at ZQN at 1900 ? (except maybe a few kiwis from AKL, WLG or CHC) departing after work that day. Does help departures from Australia.
It would help delays though. At present I think aircraft MUSt depart 45 mons prior to sunset. If eg. a delay at OZ end, aircraft could still take off with knowledge of no effective curfew.