Is there anyone on the Coast who does this?
Sometimes the honest answer is that the nearest is in Christchurch or Nelson, and the quote should say so with travel in it. NovaPay asks locally first and tells you when it has had to look further rather than quietly widening the search.
Why did my deck membrane fail?
Usually movement, ponding water or a detail at an edge or penetration rather than the membrane itself. That is why a quote for recoating without looking at the falls and flashings is one to be careful with.
Is this the same as what a tiler does?
Overlapping. Tilers routinely waterproof showers as part of tiling. A dedicated waterproofer is who you want for decks, balconies, basements and anything that has already failed.
Will it be certified?
For a bathroom it must meet the building code and you should get documentation. For a deck, ask what warranty covers the membrane and what covers the labour, since they are often different lengths.
Do the tiles have to come up?
For a failed membrane under tile, usually yes, and any quote that says otherwise deserves a question. That is most of the cost, which is why the investigation is worth paying for first.