A police spokeswoman said they were called to a premises in Littlebourne Rd about 3.15pm after reports of a "grievous assault".
An injured person was found and taken to Dunedin Hospital.
Police also located the alleged offender, a 39-year-old woman, who was taken into custody without incident.
This morning, Senior Sergeant Anthony Bond said a 7cm blade was used in the alleged unprovoked assault.
The two people were unknown to each other, he said.
The victim was taken to Dunedin Hospital with a stab wound in her thigh, Snr Sgt Bond said.
Moana Pool patron Simon Wright said he was entering the facility when a person came running into the reception area and said someone had been stabbed outside, asking staff to call the police.
When Mr Wright went outside to have a look, he saw a woman - who he estimated was in her early 40s - holding a short blade that looked like a pocket knife and another woman limping away with a leg wound.
"I approached the offender and I was basically telling her to put the knife down," Mr Wright said.
He was worried she was about to go on a "stabbing spree" because she looked agitated, he said.
Mr Wright and the woman raised their voices at each other, then an unmarked police vehicle pulled over and an officer began negotiating with the woman.
"He ... managed to hold her and a patrol car showed up and she was apprehended."
Mr Wright said he did not know where the woman came from and only witnessed the end of the incident.
"I didn’t want to be a hero or anything but I just wanted to do something.
"I wasn’t scared, I just didn’t want anyone to get stabbed.
"I just thought if I walk away she is going to stab someone."
When Mr Wright went back to the pool reception area, the woman who had been stabbed was being assisted by police and Hato Hone St John.
A witness, who was inside the facility at the time of the incident, said he heard a scream outside.
A large police presence could be seen outside the pool and the road was taped off yesterday afternoon.
A Hato Hone St John spokeswoman said they responded with two ambulances and treated one person with moderate injuries, who was then taken to hospital.
A Dunedin City Council spokeswoman said the organisation’s thoughts were with those impacted and its focus was on supporting its staff.
The woman appeared in Dunedin District Court this morning.
She was granted interim name suppression and remanded in custody to reappear later this month when she was expected to enter a plea to charges of wounding with intent to cause grievous bodily harm and possessing an offensive weapon.
The maximum penalty for the wounding charge is 14 years' imprisonment.
