WWOOF in each country is a separate organization. You have to join the one you want to use. In New Zealand, contact
The recommended stay is 2-3 nights: long enough to get to know your hosts, but not enough to become a burden. You can negotiate the length of the stay with your host once you've met, however.
To join, you pay them some money ($55), write up an information sheet on yourself, and get interviewed by a host that lives in your area. The interview is no big deal; if you're an openminded traveller interested in other cultures, you're there. To get a hostlist, you send them a $25 deposit. You get the money back when you send them back the list. They are more concerned about the ownership of their list than WWOOF is, since they screen their hosts and travellers, and they want only the people they have screened to actually use their lists.
SERVAS in the US can be contacted at: