If you join the New Zealand Historic Places Trust Pouhere Taonga they have a reciprocal arrangement with the National Trust allowing free entry into sites in the UK.
Join online and they will post your membership card to you in the UK (or wherever you live). The price of £31 is the approximate conversion of $79 NZD for Family membership including $10 NZD for postage.
I think this may have been posted here before but I couldn't find it when I looked so it may have been elsewhere.
* Family - 2 adults with/without children at school - $69
* Individual - $64
* Joint senior citizen - retired person 60 years or over plus one other person at same address - $50
* Individual senior citizen - retired person 60 years or over - $45
* Full time student/unwaged - $45
Visiting Rights to Overseas Heritage Places
Members of the Historic Places Trust are entitled to free admission* to hundreds of historic properties around the world as a result of reciprocal visiting agreements in place between the Trust and other overseas heritage organisations.
Please note that the organisations listed here do not necessarily operate all of the heritage properties in the country where they are located. Some heritage properties are operated by organisations with which we do not have visiting agreements and therefore will charge you admission. Membership Services can provide you with a list of the properties to visit which are covered by our visiting agreements - call our freephone 0800 802 010.
Members can visit properties operated by the following overseas heritage organisations free of charge* upon presentation of their Historic Places Trust membership card.
*Surcharges may apply for parking, use of other amentities and at times of special exhibitions and events.
National Trust of England, Wales and Northern Ireland
National Trust of Scotland
National Trust of Jersey
National Trust of Australia
Georgia Trust for Historic Preservation (USA)
Japan National Trust
Zimbabwean National Trust
Gelderland Trust for Historic Houses & Gelderland Trust for Natural Beauty (Netherlands)
National Trust for the Cayman Islands
Bermuda National Trust
Manx National Heritage
Barbados National Trust
National Trust for Heritage Preservation (USA)
Note: free admission for NZHPT members does not apply at all properties.
NZ Historic Places Trust members receive the same preferential rates offered to National Trust for Heritage Preservation members at participating Historic Hotels of America via this website.
Note: conditions apply - rates vary, are subject to availability, and advance reservations are required; blackout dates may apply; the member rate must be requested; the member rate is not applicable to groups nor to be used with any other offer; a valid membership card must be presented upon check-in or the hotel may charge full rate.