A car cannot be issued a valid CSC as the vehicle has to primarily designed for sleeping in. So the question is not how can it have one, its who issued it, and more importantly, what can be done about it. In the absence of a controlling government department, best course of action would be report such concerns to the NZMCA, who can take appropriate action.
To those who decide they see a sticker and think it cannot possible be issued legitimately, what skills and authority do you have to decide this? Do you apply those skills fairly and without bias to all vehicles you see with a CSC sticker. Do yo also check there WoF and EWoF while you are at it? The NZMCA president recently asked members to avoid this behavior for some very good reasons.
All that said, I think there is a more important point - the unwavering reliance most have on CSC as a way to decide if the operator is abiding by the rules is fundamentally flawed.
Don't be alarmed when you see me talking to myself, I am getting expert advice.