According to recently published 2018 census data, there are 20,061 people identifying themselves as JWs in NZ (see this file, table 26). For comparison, in 2013 this figure was 17,931, in 2006 - 17,910, and in 2001 - 17,829 (source; see this file, table 30). The peak publishers figure for 2018 was 14,301.
I guess this surprise change can be partially attributed to changes in census methodology.