A woman endured a year of pain after a doctor inserted a new conceptive device without removing the old one first.
The unnamed GP says she "sincerely regrets" the error which landed her two breaches and two adverse comments from Health and Disability Commissioner Anthony Hill in findings released this week.
The report reveals that the woman, a fit and healthy 42-year-old, went to her GP in 2012 to discuss heavy periods.
She says she told the doctor she had a copper IUD that was due to come out in five months' time, and it was decided she would get a new Mirena IUD to replace it, to ease heavy periods.
Five months later she went back to the same doctor who examined her and inserted the new device without removing the old one.
She suffered months of side effects, including fever, chest and arm pain, dizziness and fainting episodes, but it wasn't until she had an x-ray after a back injury one year later that the error was picked up.
During the commissioner's investigation the GP, referred to only as Dr B, claimed she was never told about the original IUD but admitted she should have inquired about her patient's contraceptive history.
In his report, Mr Hill was critical of the doctor's failure to properly assess the woman, failure to read earlier notes stating she already had an IUD, or consider other causes and solutions for heavy periods.
The doctor also failed to keep adequate notes from consultations, Mr Hill said.
The errors were "matters of individual clinical judgment" and did not reflect on the clinic where they occurred, the report states.
Mr Hill recommended the GP apologise to the woman and review her examination technique so such errors are not repeated.