The New Zealand registry offers individual .nz registrants the possibility to have their contact address masked from public display in the whois by sending an e-mail to the registry (email@example.com).
Only the domain name registrant or the admin contact can make such a request. Individual registrants will then have their contact address masked with a unique reference code and the P.O. Box address of the registry. The registry will on-forward any post received to the registrant concerned.
You can find more details on the registry website: https://www.dnc.org.nz/pamo