The Green Island resident has been in the job 10 years and taught about 60,000 children.
Her last official day was yesterday.
Mrs McMillan's job has been cut to 20 hours a week and the reduction in income prompted her to give it up.
She will train a replacement.
When contacted by the Otago Daily Times yesterday, Mrs McMillan said St John had banned her from talking to the media.
Otago Primary Principals' Association president and Bathgate Park School principal Whetu Cormick said Mrs McMillan had delivered an outstanding programme to many schools over the years and would be missed in the role.
''I think it's a real shame that has happened. Belinda will be a huge loss to Dunedin schools and of course to St John,'' he said.
St John South Island community programmes manager Chrissie Cope, of Christchurch, said the disestablishment of a fulltime Safe Kids tutor in Otago was the result of a review.
Nationwide, the number of full-time equivalent (FTE) staff in the programme had not changed, but the ''distribution'' of those employees had, to better reflect population, Ms Cope said.
''Safe Kids is a charitable activity, and St John a charitable organisation, and we are not in a position to expand the programme. We needed to look at how we deliver the programme and distribute FTEs,'' she said.
Safe Kids tutors went into preschool and primary school classrooms to teach children about first aid, St John and what to do in emergencies.
They also helped children become accustomed to medical equipment, demystifying some things youngsters might find scary.
Ms Cope said the new Safe Kids tutor in Otago would cover St John's ''Waitaki'' area, one of four in the South Island.
Otago children would not be affected by the staff change, she said.
St John wanted more volunteers to be involved in Safe Kids and hoped to increase the programme's reach regionally and nationally.
''It's not about taking away any paid hours from the service, it's about thinking creatively. We only have so much money to go around,'' Ms Cope said.
Safe Kids had been moved under St John's community programmes ''umbrella'' so should have more stability, she said.
The Otago tutor would have administrative support from St John staff in Dunedin and Christchurch.
