The 100-metre freestyler mysteriously withdrew from the 2019 World Championships in Gwangju just ahead of the competition taking place.

Originally, Jack posted to Instagram she would be leaving due to 'personal reasons'.

However, on Saturday night, a media release sent by Swimming Australia said after the organisation was made aware of the adverse test result they "immediately took action" to suspend Jack from the Australian team.

The out-of-competition test was conducted on June 26.

Under the Swimming Australia policy, any athlete under provisional suspension during an ASADA investigation cannot take part in any competition.

While all details will remain confidential until ASADA has completed their investigations, Jack took to Instagram.

 
 
 
 
 
 
 
 
 
 
 
 
 

They say a picture is worth a thousand words, in this case a picture can not describe the amount of pain and vulnerability I am feeling right now. It is with great sadness and heartache that I had to leave due to allegations of having a prohibited substance in my system. I did NOT take this substance knowingly. Swimming has been my passion since I was 10 years old and I would never intentionally take a banned substance that would disrespect my sport and jeopardise my career. Now there is an ongoing investigation and my team and I are doing everything we can to find out when and how this substance has come into contact with my body. I would appreciate if you respect my privacy as this is very hard for me to cope with

A post shared by Shayna Jack (@shayna_jack) on

"It is with great sadness and heartache that I had to leave due to allegations of having a prohibited substance in my system," Jack wrote.

"I did not take this substance knowingly."

Jack has been an integral part of Australia's 4x100m freestyle relay team alongside Cate Campbell, Bronte Campbell and Emma McKeon. 

While Swimming Australia will continue to provide support for Jack during the investigation, they remain clear on their approach to any performance-enhancing drugs.

"Swimming Australia is and always has been committed to a clean sport. We have consistently supported a zero tolerance approach to doping and have been strong supporters of a rigorous and consistent approach to drug testing both here and overseas," CEO Leigh Russell said.

"As you would expect we are bitterly disappointed with allegations a swimmer has a prohibited substance in her system although it is important to point out that the matter is yet to be determined.

"We will continue to provide appropriate support for Shayna. We will also provide support for our team members who are still in Korea and our team and our organisation will continue to reaffirm our zero tolerance approach."

Doping has been front and centre at the World Championships with Rio Olympic gold medalist Mack Horton being the most outspoken. 

His stoush with China's Sun Yang has been ongoing since Rio, where Horton called Sun a "drug cheat', and continued into Gwangju with the 23-year-old Australian refusing to take the podium or shake his hand after the men's 400m freestyle.