A young Adelaide woman on a working holiday in Colombia has been arrested with 5.8kg of cocaine in her luggage, her family says.

Cassandra Sainsbury, 22, was detained for drug trafficking on April 11 at El Dorado International Airport in Bogota on her way back to Australia.

Her sister, Khala, who has started an online campaign to raise funds for legal fees, says she is innocent.

"Cassie would never do anything like what she has been accused of," Khala writes on the Fundrazr page.

The cocaine was reportedly hidden inside 15 pairs of headphones Ms Sainsbury bought before her departure, The Adelaide Advertiser reported.

"They x-rayed her luggage and they found 5.8kg of cocaine concealed in packaging which was meant to be gifts for people for her bridal party and family friends," Khala told The Adelaide Advertiser.

Khala said in the days prior to her flight home "she was with somebody she had met that could speak English and she was sightseeing".

"He was showing her around," Khala said.

She saw some headphones she was going to buy and give to friends, and he told her he knew a guy that could get them cheaper.

"She did that and got them handed to her Wednesday morning before she left. She just put them straight into her suitcase."

Ms Sainsbury, who is a volunteer for South Australia's Country Fire Service (CFS), was in the country for a working holiday and was due to be picked up from Adelaide Airport on Good Friday.

Her family says she is facing up to 25 years in jail "for a crime she did not commit".

She was denied bail at her court appearance and is being held at El Buen Pastor women's prison, Colombia's largest women's prison, which is reportedly heavily over populated.

The family says it must raise at least $15,000 for legal costs.

"Cassie was planning her upcoming wedding to the love of her life. She has her full life ahead of her, and now its all put on the line because of this," her family says.

Ms Sainsbury's mother, Lisa Evans, said she was "scared to death" for her daughter.

"I'm devastated that my little girl is in this place. Our family just wants her home safe," she writes on FundRazr.

A spokesman for the Department of Foreign Affairs and Trade said it was providing assistance to an Australian woman arrested in Colombia but offered no further details due to privacy.

As of early Monday, almost $2000 had been raised, only 12 percent of the $15,000 needed.

AAP/Fairfax Media