Concerns are being raised over the number of mental health patients placed under compulsory treatment.

A group of lawyers, psychiatrists and mental health researchers are raising these concerns in a new book - New Zealand's Mental Health Act in Practice.

It points out some 4000 people are now receiving compulsory treatment under this act at any given time.

Professor John Dawson of the University of Otago's Faculty of Law said the higher numbers may be due to a misinterpretation of the act's criteria.

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"For instance the concept of serious danger to others, which is one of the standard governing compulsions, seems to have expanded significantly over time," he said.

"That overly conservative and risk-averse interpretation can lead to rapidly increasing numbers of people under the act."