Diversion agreements help the airport to manage the efficient use of its facilities by limiting the number of large aircraft that might turn up with little notice.
For widebodies this would make some sense and it is in the ERSA they need prior permission for unscheduled Ops. However this wasn't a widebody and is a totally different AOC and operation to QF Long Haul..
Personally I would have call the Feds to move the car and told the airport to sort it out with legal department because I still can't see how what the Airport did was actually legal.