Quoting from the Fire Service Act 1975 before it gets amended:
Section 33: Volunteer fire police
(1) With the consent of the senior constable in the district, any Chief Fire Officer, in accordance with the policy of the Commission, may establish a volunteer fire police unit and enrol any person as a member thereof and may in like manner disestablish such a unit or for just cause remove any person from the roll.
(2) Every person enrolled under this section shall be sworn in before a Justice of the Peace, and during the period of his service as a member of the fire police and while so acting shall be deemed to be, and shall have all the power and authority and responsibility of, a constable.
(3) In the performance of their duties at any fire or other incident to which a fire brigade is called, volunteer fire police shall carry out their duties under the direction of the person in charge of the fire brigade.
Compare: 1972 No 119 s 44
Section 33(1): amended, on 1 October 2008, pursuant to section 116(a)(v) of the Policing Act 2008 (2008 No 72).
The NZFS have circulated an internal policy that as the Policing Act apparently makes the Fire Service Act invalid.
Yet the Policing Act includes phrases like
(Section 49) (1) A person commits an offence who, without reasonable excuse, carries on an activity under an operating name that includes the word "“Police”" or the words "“New Zealand Police”", in a manner likely to lead a person to believe that the activity is endorsed or authorised by the Police or any part of the Police.
You've also got Section 116:
Terms relating to Police in other enactments
Unless the context otherwise requires,—
(a) a reference in an enactment other than this Act to any of the following terms must be read as a reference to a constable: