The patient said she thought her chest was "broken", telling staff: "I could not stop crying."
Mr de Bruin got his job back after taking his bosses to the Employment Relations Authority, but still faced sanctions through the Health Practitioners Disciplinary Tribunal.
After a hearing in Christchurch last month, where seven of his colleagues gave evidence, he was found guilty of professional misconduct.
In a ruling out today, a "deeply remorseful" de Bruin was told by the tribunal he could keep his job as long as he undergoes professional supervision for three years and pays $3000 in costs.
The tribunal concluded that Mr de Bruin recognised "almost immediately" after the slap that he should not have assaulted the patient.
It could not be established if he had put his knee into the woman's chest. She was checked for injuries and none were found.
The tribunal gave him credit for 42 years of prior good conduct, and noted that he had welcomed regular supervision and had undertaken further training.