guess it depends what car tho...
i flew to auck ($149 one way roughly) then drove down. i had three tanks of premium in the trueno ($60, $49 and $43) for the ride down. one way on the ferry was reasonably cheap as i recall too. was cheaper for me than using a transport company....(if your err dont count the speeding fine incurred on the late night drive past rotorua
).
might pay just to look into it?