He has worked across Australasia in clinical
and medicolegal contexts, including Auckland's Regional Youth Forensic Service. Dr Armstrong has a private clinical practice and a particular interest in criminal and civil forensic cases, with experience writing reports on issues ranging from the insanity defence to worker's compensation for mental injury. As well as experience as an expert witness giving testimony in District, Youth, Family and High Courts, Dr Armstrong is an Honorary Senior Lecturer for the Universtity of Auckland and is published in the area of fitness to stand trial in the Youth Court. He is the New Zealand representative to the Section of Child and Adolescent Forensic Psychiatry of the Royal Australian and New Zealand College of Psychiatrists.