To obtain a firearms license in New Zealand you must apply directly to New Zealand Police. The link at the bottom of this page opens a new page directly on the New Zealand Police web site.
Note that under the current government regime the police have limited resources allocated to processing firearms license applications and there can be considerable wait times between applying for a license and the license being processed. These delays also effect renewals and police current encourage licence holders to apply for renewal of their licenses four to six months before the expiry date on the licence.