That's Chatham Island. A looong way out! To be precise, 650km from the closest point of NZ mainland.
en.wikipedia.org/wiki/Chatham_IslandI'm always looking at it and thing it would be absolutely amazing for windsurfing. Less the lagoons (not sure how deep they are) but rather the countless beaches, facing any possible direction. Should be amazing wavesailing! If it weren't for the cold and it's got a reputation for BIG great whites. Flights not cheap, but would be an epic mission. Anyone keen? Im happy to organise it!