A self-contained van is a van that has been certified that it can contain water waste for up to 3 days. This includes water in the form of water supply, grey water (sink and shower water), and septic waste.
This certificate is a sign that a van is more environmentally friendly than other vans, because user have less impact on their surroundings etc.
Therefore, New Zealand rewards drivers of self-contained vans with more campsite options that only allow self-contained vehicles, as well as the option to camp on public land where permitted.
For a full list of Freedom campsites download apps like Campermate, or wikicamps.