Thinking about getting mobile broadband in NZ. You might want to check out the following map from Telecom NZ. I am not sure whether this inter-connects with Vodafone's network, so you might want to look at their coverage too. I think they are completely separate.
It is noteworthy however that coverage declines significantly in bad weather, or along the coast, so you might want to look at a fixed point broadband to the door.
I would suggest that mobile coverage is not so much the issue, as the expense of it. The capacity available for mobile broadband is highly restrictive (i.e. expensive). So the prospects of working from a campervan might be just a dream at this point....at least in NZ.