Why do you need to take this through the resolution centre @Melanie467 ?
This is a simple matter. Your guest booked your room. The room including the bed is as described. He has chosen not to stay and it is too late for you to get a replacement booking.
I would just reply and say you are sorry that he hadn't noted from the photos and description that you have a double bed (I don't know about NZ, but in Europe this is the standard size we use so is the norm). and has therefore chosen not to stay.
That if he choses to cancel that will fall in line with the cancellation policy he booked under and therefore a refund won't be possible.
As others have said regardless of whether you refund he could leave an unfair negative review.
Personally I wouldn't review this guest unless you see he has left you a review. Just keep your review factual. Something like;
"XXX decided not to stay, as on arrival he decided he would prefer not to stay in a room with a double bed. The double bed is clearly shown in your photos and it is also clearly mentioned in your description so it is unfortunate that XXX missed this when making his booking".