Jack, 20, faces a ban after making the shock announcement on Sunday via social media that her B sample tested positive to a non-steroid anabolic agent popular with bodybuilders - Ligandrol.

"She is going to fight it, we are going to fight with her and Swimming Australia is going to fight with her because I believe in Shayna," Boxall said at the world swimming titles in South Korea.

"This is a very sad story ... but we just have to go through the process, give it respect, trust it but I believe it will all be finished (by beating it) - absolutely."

Jack said in the Instagram post she had not heard of Ligandrol let alone pronounce it while Boxall admitted he "hadn't even looked it up" since the positive test.

Jack believed the positive test came from "contaminated supplements".

Asked how she could have tested positive, Boxall said: "Absolutely no idea".

Boxall was present when ASADA told Jack about her initial A sample positive test on July 12 while they were at a world titles training camp in Japan.

It took an 8km walk with Jack to calm her down before she was sent home, initially citing "personal reasons".

While devastated, Boxall said he supported SA following protocol and sending Jack home just days before the world titles started in Gwangju.

"I support Shayna, Swimming Australia and support our stance and zero tolerance for drug cheats," Boxall said.

"So does Shayna. That's why she left immediately. We followed the process."

Her legal team must now prove to ASADA that it was unintentional or accidental to beat a ban.

It was believed Jack was still waiting for B sample results after SA's shock announcement on Saturday that the 20-year-old had tested positive to a banned substance.

However, Jack posted on Instagram on Sunday that it had come back positive nine days ago for Ligandrol, which increases muscle mass.

"Seeing my parents, brothers, boyfriend and grandma made me break down into a million pieces as this was so hard for me to cope with," Jack posted.

"It just didn't make any sense, and still doesn't to this day.

"Deep down, I feel I shouldn't have to defend my reputation as I know that I didn't do this."

SA said it couldn't comment on the B sample news until ASADA announced its sanction.

SA officials at the world titles have only spoken of an A sample following confirmation of Jack's positive test.

SA CEO Leigh Russell said on Sunday she understood Jack had planned to reveal the positive test on her A sample after the world titles so it wouldn't be a distraction for the Dolphins.

Then Saturday's bombshell dropped.

Russell said confidentiality required by SA's agreement with ASADA didn't allow them to reveal Jack's test before the news broke.

Russell on Sunday described Jack's A sample positive drug test as bitterly disappointing and embarrassing for the national swimming team, the sport and the country.

In November 2018, ASADA issued a warning about the rise of Ligandrol, also known as LGD-4033, with 17 positive tests - nine in 2017, six in 2016 and two in 2015.

Australian Associated Press