The midwife and the patient's partner met by chance in mid-2013 at their local shops, drank beer at a nearby park and "engaged in kissing and fondling", the patient told the commissioner.
Within weeks a sexual relationship had started. The midwife admitted to the commissioner that the relationship and sex-talk in text messages had taken place.
Mr Hill said the sexual relationship continued until early last year.
The complainant said she had begun to suspect the relationship towards the end of the pregnancy, but didn't find out until after the baby was born, when she read text messages on her partner's phone.
"The final two months of my pregnancy were horrible," the patient said. "I was working, I had two small children and I thought I was going crazy to be thinking that my midwife and so-called friend was sleeping with the father of my children."
"I literally thought I was losing my mind. I was paranoid and in a high state of anxiety."
"I went into shock, I could not believe that two people who were supposed to care for me could possibly be so horrible and act with such disregard."
She considered changing midwife but was "too overwhelmed" and convinced herself that she had been wrong.
Mr Hill said a midwife having a sexual or emotional relationship with a patient's partner was contrary to the Midwifery Council's Code of Conduct.
His midwifery adviser, Billie Bradford, said a midwife's having an affair with a patient's partner "undermines the primacy of the midwife's professional relationship with the woman ... and is not only disempowering but also potentially detrimental to the woman's emotional and psychological wellbeing.
"In doing so [the midwife] has undermined the importance of the partner to the woman and potentially contributed to disharmony in the family unit."
The midwife's relationship with the partner was a new source - in addition to her friendship with the patient - of potential loss of objectivity.
The midwife told the commissioner her conduct was unacceptable. "All due to a bad lapse of judgement on my behalf. I am embarrassed and ashamed ..."
The patient also complained about the midwife's overall professionalism, but Mr Hill said the care provided was reasonable and in line with expected practice.
The midwife quit practice for a time after this incident, but has decided to return to work in a hospital, rather than in independent practice, and she has agreed to Mr Hill's recommendation to be mentored by a senior colleague for a year.
