A Sydney GP has been found incompetent to practise medicine because he has a narcissistic personality disorder, which impairs his ability to relate to, diagnose and treat patients.

Michael Quach's bizarre behaviour included diagnosing a 20-year-old woman who presented with vomiting and a sore throat with a middle-ear infection and sending her for a pregnancy test, even though she had assured him she was a virgin.

An investigation into Dr Quach's practice found he "dramatically over-investigated" patients' symptoms and provided "unusually extensive medical treatment" such as giving intravenous and intramuscular antibiotics and fluids.

During his time working at the Baulkham Hills Medical Centre from October 2011 to March 2012, numerous patients made complaints about his arrogance and rudeness. He told patients with heart conditions and high cholesterol that calcium channel blockers and statins were harmful and should not be used. He diagnosed a large number of patients with asthma despite them not having any history of the condition. He audio recorded consultations with patients, telling his supervising doctor Dr James Jeong it was because "they complained about him without cause".

Dr Jeong said it was difficult to work with Dr Quach "because he gave the impression that he was always right and everyone else was wrong" and "he was unable to accept feedback and he was convinced he was right in every case that we discussed".