Australian woman Cassie Sainsbury could face decades in a Colombian jail after a judge rejected her plea bargain over cocaine-trafficking charges.

Key points: Judge rejected plea deal that would see Sainsbury plead guilty and serve six years

Judge rejected plea deal that would see Sainsbury plead guilty and serve six years Prosecution has up to 120 days to prove Sainsbury's intent to smuggle drugs for money

Prosecution has up to 120 days to prove Sainsbury's intent to smuggle drugs for money Sainsbury's lawyer will ask prosecutors to drop charges due to lack of evidence

Sainsbury, who faces up to 30 years in jail, sobbed as she was led from court in Bogota this morning.

The 22-year-old was arrested in the Colombian capital's airport in April with 5.8 kilograms of cocaine hidden in her luggage.

The plea deal struck with prosecutors in July would have seen her serve a reduced six-year jail term in exchange for accepting responsibility for drug trafficking if the judge had accepted the deal.

The judge read out testimony given by Sainsbury on May 18 in which she detailed how a man in Bogota threatened her with a gun and sent her photos of her family.

Cassie Sainsbury waiting for the start of the hearing. ( AP: Fernando Vergara )

Sainsbury claimed the man, whose name was Angelo, said she was being watched and her family and partner would be killed if she did not carry the drugs.

The judge said he could not accept the plea deal because under Colombian law Sainsbury would have to admit to willingly attempting to smuggle cocaine out of the country.

The prosecution has up to 120 days to present evidence proving Sainsbury's intent to smuggle drugs for money.

Sainsbury's lawyer, Orlando Herran, claimed the judge did not believe the prosecution had a strong case.

He said he would ask the prosecutor to drop the charges due to a lack of evidence.

Sainsbury, who was supported today in court by her mother Lisa Evans and finace Scott Broadbridge, will face trial at a later date.

Drugs were hidden in headphones, police say

Sainsbury was arrested and detained at El Dorado International Airport in Bogota on April 12 after an X-ray machine detected the cocaine hidden in her luggage.

Sainsbury, whose family claim she was in Colombia on a working holiday, was preparing to board a flight to London on her way back to Australia when she was arrested.

Colombian police said the drugs were divided into 18 plastic bags and hidden within boxes of headphones, which she claimed were bought as gifts for her bridal party guests.

Jorge Mendoza, Anti-Narcotic Police Ports and Airports director, said police were given a tip-off Sainsbury may be carrying drugs out of the country.

He estimated the drugs were worth about US$1 million.

Most foreigners in Colombian jails are in there on drugs charges

Sainsbury has been detained in El Buen Pastor women's prison in Bogota since her arrest.

If Australia cannot negotiate a return for her, she will serve out any sentence there.

According to Colombia's National Penitentiary Prison Institute, 880 foreigners are detained in different prisons across the country. Most of them are serving time for narcotics trafficking.

The country's 2017 census said most of them were from Venezuela, Ecuador and Mexico.

El Buen Pastor (The Good Shepherd) is notoriously overcrowded. ( Flickr CC: District Women's Secretariat )

As a foreign inmate, Sainsbury is likely to receive special privileges.

Former inmates say foreigners are allowed unlimited phone calls, while nationals are limited to two or three.

While her family is visiting Colombia, Sainsbury may also be permitted visitors on non-visiting days.

El Buen Pastor prison, which houses 1,845 inmates, has been described by inmates as a cold cement building.

The biggest issue reported is the lack of adequate health services, which are provided by the medical brigades that visit once or twice a year.

There was no gynaecologist for the inmates on site last year, according to Colombian Solidarity Network RedHer.

Colombian Migration Authorities said once Sainsbury completed her sentence she would have 24 hours to leave the country and would be added to a permanent list of drug smugglers.

There have been 33 arrests for attempted drug smuggling out of Colombia this year alone, according to Colombia's Anti Narcotics Police.