Kawau Island sits in the Hauraki Gulf north of Auckland, New Zealand, and it is not a normal car-rental destination: the island has no real road network to speak of, no car dealerships or rental counters, and most visitors get around on foot, by water taxi, or along a handful of short unsealed tracks rather than in a hired car. If you are searching for car rental at Kawau Island, the honest answer is that there is nothing to book on the island itself, so the practical move is to arrange your vehicle on the mainland before you cross.
Most travelers rent a car in Auckland, roughly an hour's drive south, or in the smaller town of Warkworth, closer to the coast and typically twenty-five to thirty-five minutes from Auckland along State Highway 1 and connecting roads. From either point, the drive continues to Sandspit, a small harbor settlement that is the main departure point for passenger ferries and water taxis out to Kawau Island. Parking a rental car at or near Sandspit for the day, or for the length of a stay, is the standard approach, since the island itself has no facilities to receive or service a rental vehicle.
Driving is on the left throughout New Zealand, and the route from Auckland or Warkworth to Sandspit follows ordinary sealed roads through rolling countryside, generally straightforward outside peak holiday periods when coastal traffic north of Auckland can build. There are no tolls on this specific route, though New Zealand does operate a small number of tolled highways elsewhere in the country. Parking near the Sandspit wharf fills up on busy summer weekends, so arriving with some time to spare before a scheduled ferry or water taxi is worthwhile. Once you reach Sandspit, plan your mainland rental purely as transport to and from the ferry, not as a vehicle for exploring Kawau Island itself.