A Georges River Council spokeswoman said a faulty anti-siphoning valve was responsible for an incident at Hurstville Aquatic Leisure Centre.
More than 150 people were evacuated from the centre yesterday. Three people, including a child, were taken to hospital.
The spokeswoman said the centre was evacuated at about 10am yesterday after staff detected an odour caused by a faulty anti-siphoning valve in the plant room, which regulated acid usage.
"As a precaution, the NSW Ambulance Service was called to attend the site," she said.
"The fire service also attended and it was able to confirm the cause of the odour and by 1.15pm declared the centre safe."
She said reports it was an "acid spill" - the words used by Fire and Rescue NSW - were incorrect.
"Council also commends the quick action of the Bluefit staff of the centre in ensuring all customers and staff were evacuated promptly and safely," she said.
"As always, the safety of customers, community and staff is the first priority."
The spokeswoman said all plant and equipment at the centre was inspected daily.
"In this case, the spa plant had been fully functional during the previous 24 hours," she said.
"However, in this case the equipment failure and siphoning occurred overnight and was detected when the system was turned on in the morning.
"The faulty valve was replaced at 1pm yesterday.
"The centre reopened at 5.30am on Tuesday.
"We welcome the community to visit the Hurstville Aquatic Leisure Centre, enjoy its school holiday programs, take part in its health club for various interactive classes, sports activities, and fully utilise its gym and swimming facilities."
Three people were hospitalised and more than 150 people were evacuated from Hurstville Aquatic Leisure Centre yesterday after a hydrochloric acid spill.
The spill, which occurred at about 10.16am, resulted in a toxic cloud of vapour that caused visitors to the centre, including children, to be overcome with fumes.
Four Fire and Rescue NSW crews from Mortdale, Hurstville and Chester Hill, including two hazardous materials (HAZMAT) teams, raced to the scene following reports of a strong toxic smell inside the centre.
At least five children and one adult reportedly suffered breathing difficulties as a result of the fumes.
Fire and Rescue NSW said more than 150 people were evacuated from the centre after about 70 litres of hydrochloric acid leaked into a spa area of the centre just after 10am. This released a cloud of vapour into the communal area of the pool.
NSW Ambulance Service said seven crews rushed to the scene and paramedics assessed and treated nine patients.
Three people, including two adults and one child, were transported to St George Hospital in a stable condition.
NSW Ambulance Service Inspector David Stride said Fire and Rescue NSW and bystanders did a fantastic job evacuating the entire facility by the time ambulance crews arrived.
"Incidents like this can happen in a matter of seconds so it's always important to be aware of your surroundings," he said.
"When a patient is exposed to any type of chemical there's always an unknown to the extent of their injury."
Fire and Rescue NSW said the leak is thought to have occurred after an automated system within the pool's filtration system malfunctioned.
Fire and Rescue NSW hazardous materials (HAZMAT) specialists in spillage suits used equipment to monitor the air inside the centre and make it safe.
A Fire and Rescue NSW spokeswoman said HAZMAT crews worked with the leisure centre's pool technician to neutralise the spill.
The acid vapour was eventually cleared with a mix of fans and natural ventilation.
She said crews continued to monitor the air before it was deemed safe at about 1pm.
The centre remained closed for the rest of the day and was due to reopen this morning.
The leisure centre is operated by Georges River Council, which released a statement late yesterday.
"Hurstville Aquatic Leisure Centre evacuation today was caused by faulty anti-siphoning valve in the plant room that regulated acid usage," it said.
"As a precaution, ambulance was called onsite.
"The centre now restored its order and will reopen as usual from 5.30am [on] Tuesday, April 6."