The crossing from Lyttelton to Diamond Harbour by ferry only takes 10 minutes. If you are taking the Ferry you will us located at B Jetty in Lyttelton Harbour. After coming through the tunnel turn left onto Norwich Quay. The second turning to the right is the Oxford street overbridge, you cannot drive over this so the best option is to park on Norwich Quay, walk over the overbridge and follow the footsteps on the pavement to Black Cat ferries. This is only a 5 minute walk from Norwich Quay.
Driving to Diamond Harbour from Christchurch takes roughly 45 minutes. The drive itself is very pictuersque, taking you around the Banks Peninsula coast line.