Dr Teo appeared before a Health Care Complaints Commission inquiry in Sydney on Tuesday to defend surgeries he performed to remove brain tumours in two women, from which they never recovered and died soon after.
Dr Teo said he believed he had been "too aggressive" in attempting to remove a tumour in one woman.
"I think that I've probably gone across the midline ... one to one-and-a-half millimetres … but that's enough to damage a patient when you're doing surgery in this area," Dr Teo said.
None of the experts who gave evidence could agree exactly where Dr Teo had impacted the brain that damaged the patient to the point they never recovered.
"Clearly I've gone too far somewhere," Dr Teo said.
"Clearly somewhere there's been a millimetre transgression."
In the case of the other woman, Dr Teo said her pre-surgery brain scans showed a displacement of brain tissue, known as a midline shift, which placed her at risk of death at any moment.
"This patient could have died at any time. Within minutes of the scan, or days or weeks of the scan," Dr Teo told the hearing.
He defended his decision to operate on the woman after other surgeons had declined.
"I've made errors in the past where I haven't appreciated the urgency of a midline shift and they've died," he said.
"I would rather err on the side of caution now and say 'you could die at any minute'."
Dr Teo said he conducted a partial right frontal lobectomy and unknowingly likely removed a part of the brain that was crucial to the woman's functioning.
"I agree I took out a bad part of the right frontal lobe that caused the deficit."
Dr Teo argued he did not remove an excessive amount of the patient's right frontal lobe - an area of the brain considered responsible for higher level functions such as language and speech.
He said there are brain networks in the right frontal lobe he didn't even know about at the time of the surgery that are not widely considered in "mainstream" neurosurgery.
"I think you could have avoided this terrible outcome with the knowledge we have today," Dr Teo said.
"I will not do a right frontal lobectomy now without mapping those networks. It's not mainstream, but it should be."
After Monday's hearing, Dr Teo told reporters the disciplinary hearing had taken him out of the operating theatre.
"I haven't been able to save lives that I know I can save," he said.
One of the main issues to be decided by the panel of legal and medical experts is whether the women and their families were adequately informed of the risks of surgery.
An account by one of the patients' husbands was read to the court on Monday, stating he was not told the surgery could go as badly as it did.
"We were told he could give us more time. There was never any information about not coming out of it," the husband said.
"We paid $35,000 for my wife to die. I never got to say goodbye."
The hearing is expected to wrap up on Wednesday before a decision is made about whether to impose restrictions on Dr Teo as a surgeon.
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