A Health Care Complaints Commission inquiry is investigating the circumstances surrounding the operations, including concerns patients were offered false hope regarding their chance of survival.
Appearing before the commission in Sydney on Thursday, the 65-year-old disagreed with fellow neurologists Professor Bryant Stokes and Associate Professor Andrew Morokoff, who told the inquiry there was no benefit in operating on one of the patients.
Dr Teo differed to them on whether one of the patient's tumours had spread to the left side of the brain, saying in his opinion it had not, making it operable.
"My opinion was this was focal. Their opinion was this was diffuse. When I did this case I was hoping that I was right," he told the inquiry.
"I even said to the family, 'If I've got it wrong it could be terrible. If I've got it right it could be great'."
Dr Teo was grilled by the commission's counsel Kate Richardson SC about why he disagreed with the evidence of the other experts.
He said he would not expect them to know as much about brainstem tumours as he did because it wasn't their specialty.
"It's not just reading a brain scan, it's the nuances of a 'sub-sub-speciality'," he said.
"I have more experience with brainstem tumours and surgery thereof than almost anyone in the world."
At times the controversial surgeon became frustrated by Ms Richardson's questioning.
"I don't blame you for being confused ... but it's what I do for a living," he told her.
The inquiry heard Dr Teo told the patient she had a five per cent chance of a devastating outcome such as death, locked in syndrome, or complete paralysis.
"Five per cent risk of a bad outcome, means a 95 per cent chance of a reasonable outcome," he said.
"The only reason you would not go home is if she had a bad, devastating outcome - which she did. That five per cent."
The renowned surgeon said he naturally reflected on his methods after having his judgment questioned by others
"When you judge a tumour as being surgically resectable and you get a bad outcome there is no one to blame but yourself," he said.
"Of course you're going to reflect on your rationale."
In the case of the other patient, Dr Teo told her if she didn't have surgery by the following Tuesday she would be "f***ing dead by Friday", her husband told the hearing.
During the 2019 surgery, the woman received a frontal lobectomy in which a significant portion of her brain was removed - something her husband told the committee was not disclosed before the operation.
Medical consent experts Paul Komesaroff and Chris Ryan told the hearing risk needed to be conveyed to patients to allow them to make informed consent.
On Wednesday, Assoc Prof Morokoff, Prof Stokes and another neurosurgeon, Professor Paul D'Urso, told the inquiry it was common during operations to remove healthy brain tissue without first informing the patient.
"I think every brain surgeon in the country would be guilty of not declaring they take out normal brain tissue when they remove a brain tumour," Prof D'Urso said.
"You'd have nothing else to do other than have neurosurgeons in your commission if that was the line you took."
Dr Teo was restrained by the NSW Medical Council in August 2021 from operating without the approval of another doctor after an investigation by the state's health care complaints commission.
He has repeatedly denied any wrongdoing.
