Immigration authorities in Australia have belatedly admitted they directed an offshore detention centre not to distribute a huge shipment of "Freedom" brand muesli bars, but are refusing to say why the directive was made or what other brands are blacklisted.

In January, the ABC revealed the company running the Manus Island detention centre - Transfield Services - had refused a large shipment of muesli bars, apparently because the Freedom brand was considered inappropriate to give to asylum seekers housed at the facility, who are locked up.

The Department of Immigration and Border Protection at the time flatly denied they were behind the ban.

"Any assertion that the Department directed the service provider not to accept the bars is wrong," the department said in an email on January 30.

But after a swathe of documents were released under Freedom of Information, including some that appeared to show the department directed Transfield not to distribute the bars, there has been a backflip.

The statement provided to you in January that the Department did not direct the service provider to reject 'Freedom muesli bars' was incorrect. The statement was made in good faith based on the information we had at the time. Nevertheless, the information provided by the Department was wrong and we apologise for the error.

- Email from Department of Immigration and Border Protection

In follow-up questions, the ABC asked the department why a brand called Freedom had been banned, whether any other goods with similar names were likewise banned and whether there was a list of banned terms available.

"We have nothing further to add to our response," a spokeswoman for the department said.

In the documents released under FOI, one person, whose name was redacted, appears to indicate other goods may be disallowed from sale at Manus Island.

"Just don't mention the container of 'Liberty' snacks ready to go out," it said.

The FOI documents also indicate Freedom bars were removed from circulation at the Nauru detention centre.

An anonymous tweeter indicated that Freedom sanitary napkins had also been banned at offshore detention centres, which the department denied.

"In relation to the sanitary napkins, no such items have been distributed at the Nauru Regional Processing Centre," the statement said.

"No direction was provided by the department or by its service provider to ban the supply of any specifically branded sanitary napkins."

Recently introduced laws make it illegal for people working at immigration detention centres from revealing certain information, a crime punishable by up to two years' jail.