You can just about forget VAA operating cover flights, they can barely cover their own schedule. Then there are only certain routes they can operate (without changes that would take time to insurances etc). So on the day, they may be able to run a NAN service (if they can crew it, and remember they need to fly with 2 sets of cabin crew on any service over 9:45 block time). Air NZ would also only be able to cover maybe a Tasman or two tops. The alliance only extends to trans-Tasman so any island flying would be full charter, and if left to the last minute, would be very difficult to arrange.