This is not legislation, but rather a professional standard or code of practice. It can be referenced as a website. For example:
New Zealand Nurses Organisation. (2010). Code of ethics. Retrieved from http://www.nzno.org.nz/Portals/0/publications/Code%20of%20Ethics%202010.pdf
OR if this source is in hard copy, i.e. a booklet (referenced the same way as a book):
New Zealand Nurses Organisation. (2010). Code of ethics. Wellington, New Zealand: Author.