Ms Potter said she first became concerned after Ms Johnson wrote, "all grand adventures end with a big bang" on Facebook on the day she was due to leave Bangkok.

When another friend asked what happened, Ms Johnson said "some pretty serious stuff went down," and said she would be flying back into Australia on Saturday January 12 instead.

However, she did not return, and sent Ms Potter a string of desperate messages on Monday night, starting with "help me".

When Ms Potter asked what was going on and where she was, Ms Johnson responded by saying she was in an immigration detention centre but she couldn't talk because her phone was being taken from her.

"So scared ... have to go," she wrote, asking Ms Potter to tell people what was going on.

Ms Potter, who works in crisis PR, said her friend would not have asked her to contact the embassy and the media if her situation was not dire.

"Alarm bells rang for me because she knows what I do and she wouldn’t put me in that position if it wasn't serious," Ms Potter said.

Ms Johnson describes herself on Facebook as an "international medical tourism promoter, photographer, freelance journalist, world traveller".

Ms Potter said she believed Ms Johnson, who had recently spent a year travelling and was due to move into her apartment in Sydney, may have overstayed her Thai visa accidentally.

"I had a feeling something was going wrong around the date that was posted on Facebook," Ms Potter said.

"She called me and asked me to wire her some money, she said it’s no problem, just a problem with immigration."

She transferred Ms Johnson more money two days later, but then did not hear anything from her until Monday night.

In a statement, a spokeswoman from the Department of Foreign Affairs and Trade confirmed the department had been in contact with Ms Johnson.

"The Department of Foreign affairs and Trade is providing consular assistance to an Australian woman detained in Thailand in accordance with the Consular Services Charter," the spokeswoman said.

"For privacy reasons we are unable to provide further details."