Dr Pragati Gautama, who works at the Cromwell Medical Centre, said she was "unhappy with the state of affairs'' regarding overnight emergency service in the town.
The area needed more paid staff, as single-crewed ambulances were common.
"The community has often been left without any local ambulance cover overnight,'' she said.
"An ideal way forward is to get real input from the community as to what emergency care in their area should be like.''
The town was "owed'' a level of transparency around exactly what was being provided, she said.
Cromwell was served by an Alexandra-based ambulance overnight, she said.
St John assistant director of operations Derek Liefting said this was not the case.
"We have local ambulance staff in Cromwell 24/7, supported by local volunteers.
"There is less chance of getting a single-crewed response at night than in the daytime.''
Cromwell was "very well served'' by its emergency ambulance service, he said.
"St John works closely with the local health community to achieve this, and we constantly monitor and evaluate our performance.''
It had eight paid staff and 18 volunteers in Cromwell.
Single-crewing was uncommon, with 89% of all responses fully crewed, and protocols were in place to ensure in these cases there was backup from a second ambulance or a local health worker, he said.
"St John is committed to ending single crewing and has significantly reduced the number of single-crewed responses over the past few years.''
Response times in Cromwell were "well above the national average'', he said.
"In Cromwell, 80% of critical or life-threatening incidents are reached within 12 minutes, and 97% within 30 minutes.''
Patients whose emergencies were not critical or life-threatening might wait more than 30 minutes, as was the case throughout the country, he said.
Emergency vehicles from other regions were used occasionally when it was appropriate for the incident, he said.
The Alexandra-based ambulance attended 11% of emergency incidents.
