The Mental Health Foundation says systemic failures may have occurred in mental health services in the Hutt Valley.
The Ministry of Health is holding an independent inquiry into a period covering a few years until 2009.
The inquiry was ordered last June under section 95 of the Mental Health Act.
Mental Health Foundation chief executive Judi Clements, says the use of that provision seems unusual.
The ministry and Hutt Valley District Health Board say they believe the problems have now been resolved, although the inquiry will establish whether that is the case.