Waikato DHB said it was now contacting about 25 patients the man saw during his tenure at the health board.
The DHB said it and the Medical Council of New Zealand identified concerns about the documents the man presented - which may have belonged to someone else.
"As these enquiries raised serious issues of potential identity fraud, this person is no longer employed by the Waikato District Health Board and is unable to carry out medical duties in New Zealand," the DHB said today.
The man was given name suppression at Hamilton District Court yesterday and remanded in custody.
The man worked in the crisis, triage and home-based treatment services.
"These services are responsible for triaging all contacts with mental health services and directing them to the service best placed to meet their needs. This person worked within a multi-disciplinary context and usually saw patients in the company of another member of staff," the DHB said.
"We are extremely concerned and our first priority is for the safety of our patients and their families. The information to date has not identified any harm to patients and we will continue to assess the situation" Dr Murray said.
"Whilst most patients seen by this person have subsequently been reviewed and are receiving appropriate care and treatment we have identified approximately 25 patients who have not been further followed up," he added.
"Each patient will be offered follow-up services as soon as possible."
The health board said it carried out checks on the man's credentials and information available to the DHB and the Medical Council "verified the person's qualifications and ability to practice in New Zealand."
Now, both organisations said they would review their systems to strengthen their ability to identify potential fraud.
The DHB said patients seen by the triage, crisis or home-based treatment teams during this time are asked to phone the Triage Service 0800 50 50 50 or email mentalhealth@waikatodhb.health.nz.