自问自答了。 找到信息了。

Can I take a rental car on the ferry?

Interislander carries most vehicles including rental cars.  Please note that Avis, Budget, Europcar, Hertz and Thrifty do not allow their vehicles to cross so they have depots at both Wellington and Picton Terminals for you to uplift another vehicle.  If hiring from one of these companies you should book yourself on Interislander as a foot passenger.

All other reputable rental car hire companies allow the vehicle to cross on Interislander.  Talk to them about the favourable rates they office to book you and the vehicle on Interislander so you can simply drive on and drive off at the other end - retaining the same vehicle throughout your journey.