Not too sure mate, I know there is a Cell for Vodafone at Matiatia but wasnt aware of any 2degrees cells there? The signal was fluctuating while I was stationary which gives reason to believe it that it may have very well been installed on a ferry traversing the Gulf
Cellsites are usually fed by Digital Microwave Radio, or Fibre Optic Cable, neither of which will work too well on a ferry which is rolling around with each wave. So I would very much doubt that it's onboard the ferry, more likely at the terminal on Waiheke Island, possibly co-sited with the Vodafone installation.
Going slightly O/T, I think the cook strait ferries (well the interislander) won't be any time soon. They have on board wifi and charge you a rather large premium for what I presume is satellite back haul so you can get coverage inside Tory Channel. Vodafone at least have a cell site on the north side of Cook Strait to provide coverage for most of the path of the strait part of the journey.