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

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

Her sister, Khala Sainsbury, says the 22-year-old personal trainer and volunteer firefighter is facing up to 25 years in jail "for a crime she did not commit".

The family has started an online campaign to raise funds for legal fees.