Keeping an open mind, I decided to look up some things for myself.
According to Airways, ECT on the 22nd of June was 1744 local. The media reports put the departure time at 1725ish. Outside of the ECT -30 limit yes, but still daylight. Is the 30 minute limit regulatory or self imposed?
The AIP Supplement puts ATC on duty until 1800. If I can remember correctly, a tower closing off greater than 20? minutes early requires a NOTAM to be put out advising of the early close (sure a Controller will correct me if wrong). The Poindexter in me also found this gem in the AIP.
NZQN AD 2 - 31.1 makes mention that ATC must be in attendance for CAT B and C arrivals and departures. Considering the above I would be surprised to find the tower was off watch.
If they have broken the rules I'm sure it will come out in the wash and appropriate action will need to be taken however, as anyone in any industry dealing with reports from the public knows, eye witnesses are well known for being extremely unreliable especially when time has elapsed from the event.