I live in Wellington and have done both the flight to blenheim as well as the ferry. The flight is on a tiny plane and only flies if the weather is ok. It is like being on a sightseeing flight and is well worth while. I would arrange transport at the other end before you go - maybe the car rental company would pick you up. I would fly one way and ferry the other way. Be aware that if we get one of our very infrequent fogs or the wind is high that the plane won't go while the ferry probably will. The weather has to be really really bad for the ferry to not go.