I've only used the intercity bus once to get from Nelson to Greymouth to catch the Tranzalpine train back to Chch about this time a year ago.
Bus was clean, kept to the published timetable, driver pointed out a few points of interest along the way. Not my preferred way to get places, but nothing wrong with it. Bus I was on had no onboard toilet, but had stops every couple of hours.
Akl to Paihia is around 4 hours by bus I would guess, so probably just a meal/toilet break about halfway along the journey.