The GP told the HDC the ill child struggled and kicked her in the stomach as she examined his left ear and eye.
"In a shock reflex, she tapped Master A's cheek with her fourth and fifth fingers," the HDC heard.
"She said that it was a light tap and not a forceful motion, and without an intention to harm."
The GP said she apologised immediately after the incident.
But the child's mother disputed this version of events, saying: "During his check over, my son became [frustrated] and packed a tantrum where he flung his arms and legs and kicked doctor [Dr B] where she then slapped him across the face."
Contrary to the GP's account, the child's mother said the incident occurred after the GP had examined the toddler.
She said her son kicked the GP's leg, instead of her stomach, and in response the GP rolled her chair forward and slapped the 2-year-old across the face.
"The slap was loud and shocking," she told the HDC.
Ultimately, Health and Disability Commissioner Anthony Hill said it wasn't necessary for him to make a finding on the precise nature of the contact, as any contact to the child's face, unrelated to a clinical assessment, was "a serious departure from accepted standards".
"It would not be unusual for [the GP] to experience irritable or unco-operative young patients when they are unwell," he said.
"Whilst I do not condone any patient kicking their doctor, I expect appropriate mechanisms to be in place to respond to this, if and when it occurs."
The incident was "unwelcome and unprofessional", Hill said, and the act eroded the trust that until then the child's mother had in the GP as her son's doctor.
Hill found the GP to be in breach of two rights of the Code of Health and Disability Services Consumers' Rights (the code), as she did not treat the child with reasonable care and skill, and, her physical contact with the child's face was disrespectful.
The GP has provided a written apology to the mother and has undergone further training since the incident.
Hill noted that the incident appeared to be out of character for the GP, saying he didn't
consider the incident to be reflective of her general competence, or to be likely to occur again.
But nevertheless, the GP was an adult medical professional and the patient was an unwell 2-year-old child.
"I consider that Dr B's response to Master A's behaviour on 19 September 2018 was inappropriate, and I am critical of this," Hill said.
"She plainly should have known better, and, as she has acknowledged, utilised many possible strategies available to her to prevent the appointment escalating to this point."
He reminded the GP of her responsibility to her patients and to herself and to ensure that she is monitoring her own wellbeing and workload.
Hill was critical of the medical centre the GP worked at, as she had been double and triple booked throughout the day, covering for another doctor, dealing with some particularly complex cases and had not taken a break throughout the day.
"Whilst I accept that clinical staff have individual responsibility for managing their workload and wellbeing, I am concerned to read about the pressures Dr B was under on the day in question, and I consider that the medical centre should have taken remedial steps earlier."
Since the incident, the medical centre had implemented a new system for managing workloads, has placed greater restrictions around double and triple booking of doctors, and has adjusted workloads for its doctors, it told the HDC.
