Chin Loh, a pharmacist at Antidote Gardens, said he heard a scream and immediately ran outside to see what had happened.
He said a customer’s dog had "lunged" at another customer while they were waiting outside and maintaining social distancing, about 11.45am.
The dog had "clamped" on to the woman’s thigh.
Mr Loh, the owner of the attacking dog and another passer-by struggled to pull the dog off the woman.
"We struggled for quite a few minutes before the dog let go, but we were trying not to aggravate it any more or cause any more damage."
When it let go, Mr Loh put pressure on the woman’s injury and dressed it with items from the chemist.
"The first thing to do was to keep her safe and ourselves safe."
He said the woman had remained calm throughout the incident.
A police spokeswoman said they were notified of an incident involving an animal and were speaking to the informant about what happened.
St John did not attend.