If you take the bus, do allow a half hour of potential WAIT time at the bus stop, plus time to get to the resort (at least 45 minutes, if you can). If the timing with the bus is good, it might only take 15 minutes, but if the timing with the bus is bad, it could take 45 minutes. After the parade, the wait will probably be under 30 minutes, but it could be 20. Also, don't be particularly slow about leaving after the parade is over. It is, in my opinion far better to get to AKL a little early, than late. There is plenty to see at AKL while you wait. After a day at AK, you may just find yourselves relaxing a in the comfy couches in the lobby (after you check in), and you can always check in early.
Come to think of it, if you can, it might make sense to try and get 5:45pm instead.