MTG,
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Every September, my wife and I rent an RV from Tui Campers in Christchurch for a 2 week trip around the South Island. Their vehicles are a little older than most and as a result are really affordable. I have always had very good experiences with them and as a repeat customer they give me an even better deal than usual (Kiwis are great people and I know they appreciate the loyalty). Not sure what you need exactly by way of a vehicle but I now ask for the smallest platform they can provide that still has an inside shower and on-board flushing toilet .... so last year, that was a 2 year-old, high-roof, diesel, Ford Transit Van. In 2015, IIRC, I had an '08 Ford Transit platform with a typical flat-panel, wide-body motorhome on the back....definitely more comfortable and roomy inside for us but that motorhome was way too scary running up the skifield roads as it was far too wide, high, under-powered and had tons of body roll. The newer van was like a sports car by comparison which resulted in an embarrassing speeding ticket.
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We never stay in campgrounds like most do ...... as you probably would know, the free camping on the South Island is exceptional. We usually take some of our own gear like small cargo nets and lots of alloy carabineers for hanging stuff, a plug-in inverter, multiport cig socket adaptor, GPS, etc . We also take our own feather-down quilt so we're cosy at night in the snow country ....it packs down to nothing and I would have to say that the supplied blankets are never warm enough in the winter. We also fly over with soft luggage that you can stow away easily once you've unpacked into the van.....that said, Tui will store any hard luggage back in the Christchurch depot if you really need them to.
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One thing that you should check on no matter who you're getting info from is what roads you are restricted from travelling on. Many won't let you run up the ski fields tracks and a lot of dirt roads. Tui had very little restrictions.
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Happy to help further if you need any contacts. I'll be back there again in 87 sleeps but who's counting??
Regards John