Those who are speculating about B777-300ER's being purchased in the future to replace the B744's as they retire - have you thought about the fact that GE stitched up an exclusive deal to power the B777-300ER and B777-200LR. If the 300ER was on the cards, I would have thought that they would have specified the GE90 engine to power the 200ER's that they have ordered so that they would have some comonality (especially as Air NZ have been a fan of GE high bypass engines for many years- eg DC10's). I believe that they ordered the RR donk because the 2 Trent 800's are 4.3 tonnes lighter than 2 GE90's and that equates to a lot more payload on every long sector.
Then again, who knows what Air NZ management will do in the future. GB