I may be cynical but is it a case of shifting the problem to someone else rather than dealing with it at its source once it became obvious curfew restrictions couldn’t be met?
Sure, there is the diversion cost (did they get to WLG and then divert or divert en-route when the penny dropped?) but QF avoided the greater cost of accommodating the pax in SYD. Once pax have been accepted for a flight and gone through CIQ, it is my understanding that the airline is responsible for them. Hotel costs in SYD for possibly 100 plus pax wouldn’t be cheap (and then also having to clear the pax back into the country if the flight is scrubbed) - much cheaper and convenient to handball the problem to the Kiwis. After all, sleeping on an airport terminal floor costs QF nothing.
They may well “apologise for the inconvenience” but do they really care, I mean REALLY care?