FIVE children and six adults are recovering from burns to their mouths and throats after a salt shaker containing caustic soda was sprinkled over their food at a West Australian fast food restaurant.

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

The powerful cleaner was sprinkled over chips served at a Chicken Treat store in Bunbury, south of Perth, leaving five children under 10 with burns.

Six adults were also harmed after eating at the restaurant, with a woman in her 20s and a man in his early 30s so badly affected they were transferred to Perth for further treatment.

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.

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.

Dr Hooi Ee, the head of gastroenterology at Sir Charles Gairdner hospital, where the two patients remain in a stable condition, said the ingestion of caustic soda could be fatal.

"If small amounts are taken, it may not even be noticeable but if a significant amount is taken then it can be very severe, up to death," Dr Ee said.

"It is like a burn, so you can get extreme scarring, and a narrowing of the oesophagus to the point where it might need operating on. But the vast majority will heal naturally."

The nine other people injured were released from hospital on Wednesday.

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.

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

"Eleven people, including five children aged under 10, presented to Bunbury Hospital following possible contact and/or ingestion of caustic matter," said Grace Ley, the South West Regional Director regional director of WA's Regional Country Health Service.

"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."

The store was situated close to Bunbury Regional Hospital, where the majority of patients were treated. It remained closed on Thursday.

"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," a company statement said.

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