I came across this article which covers someone testing Global Xpress for Aviation aboard a Lufthansa flight a few weeks ago. They were getting speedtest results in the range of 1.4Mbps – 14.2Mbps. This was able to deliver consistent YouTube video in High Definition (720p) without interruption.
I had heard of potential speeds quite a bit faster than that – but when you consider the total speed is split between all active users on the flight I would imagine those results could be fairly normal on Air New Zealand flights. I look forward to trying out the new Wi-Fi during the year.