FIVE children aged under 10 were among 11 people who needed hospital treatment after a visit to a Bunbury fast food restaurant left them with mouth and throat burns from caustic soda which had been mistaken for salt.

Six adults were also injured in the incident at a Chicken Treat outlet in Bunbury, south of Perth - including two who were flown to a Perth hospital for further treatment.

They are a woman in her 20s and a man in his early 30s.

Three children remain in hospital.

Caustic soda, or sodium hydroxide, is commonly used as a detergent or drain cleaner and causes chemical burns in contact with the skin, and can induce permanent blindness if it contacts eyes.

Forensic police are heading an investigation into how the caustic soda came to be in the salt shaker.

As the company went into damage control, saying they deeply regretted the incident, WA police, the Department of Health, and the local authority all began investigating how the potentially deadly mix-up occurred.

Staff were interviewed, as forensic tests were carried out at the Blair Street outlet, which remained closed today.

St John's Ambulance said the alarm was raised by WA's Department of Health on Wednesday night, and several people were treated at the scene.

A spokesman said they had received a report about 7pm that an employee had accidentally put caustic soda into a salt shaker.

A PerthNow reader said: "The hot chips they bought were covered in caustic soda, instead of salt. A young kid has made a mistake.

"My cousin and another were flown up to Royal Perth last night and my young seven-year-old cousin has burns around his lips and tongue and mouth.''

Local woman Leah Clements said she realised something was wrong after purchasing chicken and chips from the store on Wednesday evening for her three-year old daughter.

"I thought she may have had tonsillitis again but then she said 'Mum I can't feel my mouth','' Ms Clements told the Bunbury Mail.

The WA Country Health Service, in a statement, said 11 people, including five children, had been affected in the drama.

“Eleven people, including five children aged under 10, presented to Bunbury Hospital following possible contact and or ingestion of caustic matter,” the statement read.

“Six have been discharged home following assessment and treatment.

“Two adults have been transferred to Sir Charles Gairdner Hospital.

“Three children were admitted to Bunbury Hospital for observation overnight.”

Police have also confirmed that officers from its South West Detectives branch are investigating the incident.

A St John Ambulance spokesman told PerthNow the two people transferred to Perth were being treated “for what was believed to be caustic soda burns.”

South West detectives and Worksafe are investigating.

Chicken Treat apology

A statement from Chicken Treat says it ``deeply regrets'' the incident.

``Chicken Treat deeply regrets that a number of customers who yesterday visited the Blair Street store later required medical assistance. We sincerely wish all of those people affected a speedy recovery,'' the statement said.

``The Blair Street store will remain closed today while an urgent investigation into all of the circumstances surrounding the incident continues.

``Relevant external authorities have been contacted and Chicken Treat will fully co-operate with any inquiries those authorities determine are necessary.

``Whilst the circumstances that caused yesterday's incident remain subject to investigation, there is no indication that any other Chicken Treat stores are affected.''

City of Bunbury CEO Andrew Brien said: “The City of Bunbury is aware that there has been at incident at Chickentreat at the Homemaker Centre.

"The Police are conducting investigations into the matter and we have, and will continue to, work with them. The City of Bunbury’s Environment Health Services team has commenced an investigation into a potential breach of the Food Act and will carry out any enforcement action as necessary.”