The four had ordered the tuna in question from the same cafe.
Health authorities said the tuna, which was served in sandwiches, was suspected to have caused scombroid fish poisoning.
Earlier, News Corporation reported the victims ate at the Soul Origin cafe in the CBD, but authorities have not revealed where the food was purchased.
Meanwhile New South Wales Food Authority said in a statement that it has obtained samples of the product implicated for further testing. It identified the product as John Bull Tuna Chunky Style in Sunflower oil. A note of the tin advised the contents were best consumed before November 2017.
“This is minor brand catering product that is not generally available to the public. The tuna is a product of Thailand imported to Australia by a Victoria company.
The cafe replaced the tuna brand it had been using once notified of the poisoning. Health authorities said the illness is commonly linked to fish that has not been properly stored.
However, the poisoning, which causes similar symptoms to an allergic reaction, is rarely fatal, according to authorities.
“The Public Health Unit of the South Eastern Sydney Local Health District has been alerted to four suspected cases of scombroid fish poisoning associated with eating fish that may not have been properly stored,” unit director Mark Ferson said.
“The Public Health Unit has notified the Food Authority which is carrying out further investigations.”
The New South Wales Food Authority said the product was an “obscure foreign brand” used predominately in catering and the importer would be contacting all its clients today.