From AirNZ
Thank you for your enquiry regarding using the Apple watch to board your flights.
Due to the function that Apple have installed which puts the watch into sleep mode when it is turned face down, please see the advice below.
To board using the boarding pass on the watch, customers need to turn their wrist upside down to face the scanner, and then 'tap' the face of the watch once with their finger - this will awaken the watch from sleep mode and allow the boarding pass to be read, even though the watch is facing downwards.