Within the one choice of van Kiwis get there is some variation from dealer to dealer. If you look through Trademe ads, some have the 18 inch wheels,(and no spare wheel) and some the far more practical for NZ, 17 inch wheels. (with a standard 17inch spare and same tyre as others) The dealer should have informed you of this. You can get as a spare a 17inch wheel/tyre and associated cradle bits and fit to your 18 inch wheeled Cali. You'd have to get the closest tyre match to match the other 4. It's 4Motion so you want to keep the tyre circumferences as close as possible.
BTW, Continental Tyres,behind Continental VW in Newmarket used to have stacks of 16 +17 steel wheels, (without tyres) taken off T5 or T6 VW Transporters that you can buy cheap. Then after your research fit the appropriate tyre and fit to your Cali. I bought 5 genuine 17 steel T5 wheels (no tyres) for a total of NZD$62.50. $12.50 each. As someone else mentions, 16inch wont fit due to front wheel brakes in your Cali. Or just buy a more costly 18inch spare wheel/tyre,but I don't think they come in steel, only Aluminium. I would def follow it up with the dealer. In NZ, on our roads you do want a real spare wheel, and know how how to fit it.