"She came out of the laundromat and she was looking a bit funny," Service said.
"I asked her if she was okay, and then she said no and then she fell and I kind of caught her.
"We stopped her from hitting her head on the ground."
The woman was very unwell and may have stopped breathing at one point, Service said.
After alerting St John, an operator told her no ambulances were available.
"It's upsetting," Service said. "This is going to cost lives."
The woman was tended by a nurse from Aurora Health Centre and firefighters from Willowbank, before paramedics arrived about noon.
St John said it was unable to comment on individual cases, but resources were placed under stress yesterday because of a high number of calls.
"As soon as St John noticed a spike in demand for its services, it called in extra resources, including four extra ambulances and additional staff," St John coastal Otago territory manager Doug Third said yesterday.
The demand on resources at this time of year was not unique to St John and other health services in the region also experienced increased demand during winter, he said.
"St John advise the public to visit your local GP for non-urgent health concerns, call Healthline on 0800 611 116 for free advice from a nurse and for emergencies please continue to dial 111."