They are legal on private property and (since 2012-06) in our capital city, Canberra, but not on any other roads/pathways.
Side note: in New Zealand they're classified the same as mobility scooters and motorised wheelchairs, and as such are fine, bust stick to walkways when possible.
They're not legal on Auckland central streets any more; my favourite pizza place, Sal's, was recently banned from using them to make deliveries.
Side note: if you're ever in orkland, go to sals, they do fantastic pizza.