How do you get from the North to the South Island if travelling by road?
You take one of the frequent Interisland ferries that travel from Picton and Wellington. Large ferries that take cars, campervans, trucks and trains. It is a scenic trip rated by some as one of the most scenic ferry rides in the world.
The trip is 92kms and takes 3hrs. About 1hr 20 mins of this is in the Marlborough Sounds (so calm). the actual Cook Strait crossing takes about 1hr (open water, can be glassy calm or rough depending on weather). And in Wellington Harbour at the North end (about 40mins).
Food, bars, indoor and outdoor seating etc all available on board. Cost around $NZ60 for a walk on passenger and from $NZ173 for a small car, one way. (Jan 2013)
Check it all out on the Interislander website at http://www.interislander.co.nz