A woman whose three-year-old daughter suffered burns to her throat and mouth after eating fast food contaminated with cleaning product, says she was scared the girl would stop breathing.

Leah Clements bought Chicken Treat from a store on Blair Street in Bunbury last night.

As her daughter Mia started eating the chicken and chips, she complained her mouth was burning and asked for water.

"Her mouth was burning and her throat and she said she started getting lumps on her tongue so it was all just burning and I thought she was getting tonsillitis," she said.

Ms Clements then tasted the food herself and immediately ran to the bathroom to rinse her mouth.

"We took the chips back and sure enough the manager turned around and said to me instead of chicken salt for the chips it was actually cleaning product," she said.

Five children and four adults who ate at the store last night have been released from Bunbury Regional Hospital, while two more adults remain in a stable condition in a Perth hospital.

It is believed the chips were covered with caustic soda. The Chicken Treat store in Bunbury where 11 people mistakenly swallowed caustic soda. ( Roxanne Taylor )

Chicken Treat has issued a statement saying it regrets the situation and is cooperating with the police investigation into how the chemical got into the food.

The company says none of its other stores have been affected.

Sir Charles Gairdner Hospital gastroenterologist Doctor Hooi Ee says it is a serious situation.

"It comes in different degrees of severity, depending on how much has been swallowed, so if small amounts are taken then arguably it may not even be noticeable, but if a significant amount is taken it can be very, very severe all the way to death, really."

WorkSafe is also investigating the incident.

Forensic police and detectives are trying to find out exactly how the chemical got into the customers food.

Police have questioned several staff members at the Bunbury store.

The City of Bunbury says it is investigating a potential breach of the Food Act.