Joe, the semi-rural congregations I know of that have been sold are: Shannon (merged with Foxton), Taihape (merged with Ohakune which has a small congregation) and Waitara (merged with a congregation in New Plymouth. Another poster reported that there have been a few mergers in the South Island (and gave examples of Invercargill and Dunedin). There are probably others that have been sold and/or merged that I have not heard about.
I am not convinced that JWs are moving out of these areas to join city congregations as much as meeting attendance and participation is declining.
Still, the 2018 Census figures should be out early next year which typically show more people identifying as JWs than are currently active - but even across at least the last 3 census's, the self-reported numbers of JWs has declined and the peak number of JWs reported in JWs' own Yearbook in New Zealand was in 2011 - about six years ago (this kind of stagnation was unheard of in previous decades).