Registered pre-stamped envelope (PSE), printed-to-private-order (PTPO) for WELPEX '72, New Zealand National Stamp Exhibition. The envelope shows the coat of arms of  Wellington, which hosted WELPEX '72. The following information is from Ralf Hartemink's site  INTERNATIONAL Civic Heraldry:

The City of Wellington adopted a coat of arms in 1878, but it was never officially registered. An official grant from the College of Arms in London was received on 6th June 1961. 

The shield combines the arms of Wellesley Duke of Wellington with those of New Zealand. The gold cross and five silver discs come the Duke's arms, after whom the city was named. As Wellington is the capital of new Zealand it is appropriate that it should incorporate parts of the national arms - the fleece, lymphad and sheaf. 

The crest shows that Wellington is a maritime city as it consists of a dolphin and mural crown. 

The supporters are a British lion and a native (extinct) moa. 

The motto - SUPREMA A SITU - means Supreme by Position. 

Literature : Information provided by Lawrence Jones.