If you are considering car rentals, try an agency like vroomvroomvroom - excellent service! This way you can see what's available, especially 'deals' like the return one-way rental Sth Is.-> Nth Is., if possible.

www.vroomvroomvroom.co.nz

I've used them un AU but not NZ. Other people might have direct experience, but I can tell you I saw at Queenstown airport, some indy rental counters in the terminal, where they appeared to promote one-way rentals which cross the ferry... but check it out for yourself first.

Hertz will involve linking a one-way retnal to Picton, catch the ferry, and another one-way rental in the north island, from what I recall.