http://www.breakawaynz.com/display.php?SUPP_ID=1047
They offer 4x4 trucks for rental on off-road trips... I haven't used them, I don't support them but might be of use to anyone wanting to explore 'the other island' without having to worry about sorting out ferry crossings and such

Unlike most rental companies we do not limit you to just sealed roads and we ploverland no restrictions on which roads you choose to travel.
If you are a new-comer or an experienced off-road driver there are many suitable routes for you. We will provide you with maps and Ken Sibly's 4WD South Island guides.
Has anyone had any first hand experience with this company?
Az