An Adelaide woman has been arrested and detained in Colombia for drug trafficking, but her family insists she is innocent.

Cassie Sainsbury, 22, was arrested after being found in possession of 5.8 kilograms of cocaine as she waited at an airport to fly back to Australia, her sister Khala said.

Ms Sainsbury's family have described her as "naive", and said she could be facing up to 25 years in prison.

Her mother and sister have said she was given some packages that she believed were headphone earpieces, and she put them into her luggage without questioning it.

Khala said Cassie, who is a personal trainer and volunteer for South Australia's Country Fire Service (CFS), was in the country for a working holiday.

Ms Sainsbury has been denied bail and will face trial in two months, her sister said.

The Department of Foreign Affairs and Trade (DFAT) confirmed it was providing assistance to an Australian woman arrested in Colombia, but would not provide further details.

Ms Sainsbury's family have set up an online campaign to help bring her home.

In a comment on the web page, her mother Lisa Evans said she was "devastated" and "scared to death".

Khala said her sister was "planning her wedding to the love of her life" and that was now on hold.