rayonline: Sbiddle can you elaborate. If one booked in advanced when the travel time is so low season could they get that booking class for free?
Airlines work by having fare buckets/booking classes on a flight. On domestic flights these are P,K,X,G,S,L,T,W,V,Q,H,M,B,Y with each booking class having a fixed price associated with it on the fare matrix and an associated fare basis code which determines whether it's seat, seat+bag, flexi time or flexi plus.
A certain number of seats will be made available on a flight for each class, but not all classes will be available on every flight or for purchase in all countries even if they're on 086 NZ ticket stock. Amex free flight availability will be booked into a specific class, so if that booking class is not available you're not going to be able to book it.