I scored a few (3 return trips) of these $0 flights ex CHC. Yesterday I did a day trip to queenstown with one & I got a 'cannot check you in' printout at the kiosk.
Asked at the assistance counter & they tapped away on the computer & then asked me for photo ID. I asked if it was because people were giving their flights to other people to fly & she said yes, and that it had been 'rampant' with this particular grabaseat promotion. The NZ fraud team were 'working overtime' dealing with a lot more cases than usual & were treating all cases very seriously.
A word of warning for those considering it.
I never would - I like flying too much to give my tickets to other people