As others say it is indeed possible to get CW seats on BA to Sydney by planning and grabbing the two seats at T-355 days. Using BA rather than other Oneworld carriers allows use of the 2-4-1 AMEX voucher.
One issue to take into account is that when arriving into Sydney you need to collect luggage, pass through Australian immigration and then recheck onto the flight to NZ using Qantas or Air New Zealand (these are three/four hour flights on narrow bady planes and definately do not have business/flat bed seating). I did this in 2017 and it is a tiring trip, especially the wait in Sydney.
Coming back we managed to only use BA from Singapore to London (A380) and paid for Singapore Airlines between Auckland and Singapore that was perfect.