It must have changed at some point then (maybe someone on here could provide clarity).

Previously, if you went on the Air NZ site and booked say AKL (NZ) SIN (NZ/SQ) LHR you were unable to use APD. You could only use card/poli etc. APD as form of payment was not permitted on market fare multi-airline itineraries (apart from the odd one like NZ/VA and possibly NZ/CX between NZ/HKG only). These had to be purchased as award fares.