There is a probate record for Adam Hunter Groat at Archives NZ. See http://archway.archives.govt.nz/
Early newspapers record the event. "Colonist" 3 January, 1871, states a crew member of the cutter, Dawn, fell overboard and drowned. The cutter was moored in the Manawatu river. "The Evening Post", 20 December, 1870, also mentions the event, but doesn't name the victim. These papers are available on line at http://paperspast.natlib.govt.nz/
I searched using the terms "drowned" and "Manawatu" within the months of December, January, 1870/71.