North to South Island new ZealandSouth to North Island New Zealand
When you are not going to stay on the South Island, it will take you (almost) one of the days to get from Picton to Christchurch by circuit. It can be very nice if you have good wheather for your cross. The Marlborough Sounds are the perfect place to take a look at the landscape at the crossroads.
They also have much more leg room than in the small planes bouncing between the islands -) I' m just joking, the trip is so brief that it doesn't make any difference. However, with the arrival at the arrival point, the check-in, the flights, the way from the airports and also the delivery and collection of rented vehicles at both ends..... Flights will not be much quicker than the ferry).
There was no need to worry about picking up hire vehicles on either side of the boat. Anecdotal I would say it's quite fast (I think it's not a new reservation, just a new vehicle that you collect). We' ve reserved both the national and the ferries a few month in advance as well (' I would suggest for bustling December).
The flight was less expensive (we traversed the shuttle with our campervan, which was small enough to have the normal toll). However, you should also look at the passable fares, which are significantly lower.