Quick update on my question. I flew with no dramas at all. No questions at check in, and no one else showed any interest in my carryon. Someone seated near me had what appears to be a medium sized hiking packing in the overhead bin, so I probably could have gotten away with a lot more.
With reference to the Original point of this thread, it seems these flights have started to fill up: at check in AKL, the kiosk was asking for volunteers to be bumped to a later flight as my one was apparently full. While it was indeed well packed, the spare seat next to me in Y indicates that either a bunch of people didn't show up, or they automatically start asking once their sales reach a certain threshold?