The woman, who has not been identified, visited the dentist in October 2019 with a "severe throbbing toothache" in her lower left molar.
She gave verbal consent for root canal treatment rather than extraction of the tooth, but there was no written consent of this.
"There is no documentation of any discussion about the risks associated with root canal treatment," the report said.
The dentist later apologised for not informing he woman of the specific risks before the treatment.
"For this I am sorry and I totally accept this error," he told the Commissioner.
He began root canal treatment at this appointment.
But during treatment at a second appointment in December, a 5mm tip of a file broke in the canal of the woman's tooth, about halfway down the root.
An x-ray was taken and the dentist referred the woman on to his colleague at the clinic who had more experience to have it removed.
"The tooth was dressed with a sedative dressing, cotton wool, and a temporary filling, and [the woman] was advised to call the clinic if she developed painful symptoms," the report said.
The woman said the pain was "godawful" and she took pain relief that evening and night.
"[The dentist] drilled my tooth, filled it with medicine [and] gave me a script for antibiotics and painkillers, and said it should calm down, there was nothing more he could do.
"I left in more pain than I went in with."
The dentist later told the Commissioner he regretted not seeking advice from colleagues or making a referral until after the file had broken.
The dentist said the time pressure to complete the second root canal treatment contributed to the error, because a 60-minute appointment had been shortened to 45 minutes.
Almost two weeks later a more experienced staff member at the clinic told the woman he also could not remove the broken root canal file.
He recommended a referral to an endodontist for further treatment.
On Christmas Eve the woman saw an endodontist who also could not remove the file. On January 20, 2020 he filled her root canal system around the broken file.
In May - seven months after her initial appointment - the woman attended another dental clinic to have the tooth removed.
In June, her initial dentist that caused the fractured file completed ACC forms for the treatment he carried out, at her request.
The dentist told the Commissioner he is sorry for any pain he caused her.
Findings
Deputy Commissioner Kevin Allan said the dentist did not advise the woman about the risks and possible consequences of root canal treatment – information she would expect to receive.
He therefore breached the code.
An independent advisor, dentist Dr Angela McKeefry, told the Commissioner that a broken file is a known risk of root canal treatment - about a 1 per cent chance - and that risk should be discussed at the initial visit.
The dentist should have been aware of the time constraints of the 45-minute appointment and shouldn't have attempted such a complicated procedure, the report said.
"Had [the dentist] stopped drilling and obtained advice, then the prognosis for tooth 36 may have been more positive. I am critical that [he] continued to drill ... and failed to refer [the woman] for specialist advice."
The report found the clinic where the dentist worked did not breach the code.
The dentist and clinic have made a number of changes since the report.
The dentist now uses a standard consent form to discuss risks, has enrolled in endodontist courses and the clinic now obtains written and verbal consent from patients in relation to root canal treatment.
