The carriages could be one of the Tranz Alpine sets used on the cross-island main line from Christchurch to Greymouth. I certainly remember riding in an observation car like the one at the front when I rode the Tranz Alpine in 2001. Another thought is that the carriages might be some of the former British Railways (standard gauge) Mark 2 carriages sold to New Zealand in the '90s and converted to 3'6" gauge.