After 12 minutes of CPR and mouth-to-mouth, the woman had only taken two shaky breaths for herself, Mrs Larsen said.
"I felt quite confident at first but as the time went on I got more desperate ... You feel every second. It was a long time.
"It was life or death, she wasn't breathing to keep herself alive."
Mrs Larsen had done a number of first aid courses when she worked in early childhood education but had never had to use the skills before.
"We always thought our emergency would be a baby being born in our shop," she said. A St John dispatcher talked her through the process, encouraging her through her fear of breaking the woman's ribs during compressions and performing mouth-to-mouth.
"As soon as I lay her down and started compressions her colour came back.
"I didn't lose hope, I just felt desperate."
Mrs Larsen contacted Tauranga Hospital in the following days to hear the woman was on life support on Tuesday and able to breathe for herself again on Wednesday.
St John territory manager Ross Clarke said Mrs Larsen's actions were life-saving.