Matiu-Somes Island is currently closed in the afternoon and for overnight visits, due to a shortage of fresh water. We’ve had too good a summer!
It’s only open for visitors from 8:00am to 1:00pm. In weekends that is extremely limiting because the first ferry doesn’t get there til 10:20am. (East by West do offer charters if you have a large group. On weekdays you can take an earlier ferry for a $70 divert fee, ok if you share that between a group).
http://www.doc.govt.nz/news/media-releases/2016/freshwater-supply-shortage-triggers-partial-closure-of-matiu-somes-island/