MELBOURNE — Shayna Jack's shock withdrawal from the swimming world championships in Gwangju remains a mystery to the Australian team, coach Michael Bohl has said.

Jack, part of the world record setting 4x100m freestyle relay team, announced she was leaving the team on social media on Monday.

"It is with great sadness that I have to withdraw from world championships due to personal reasons. I appreciate everyone's support and patience. Thank you all," Jack said on Instagram with a picture of herself with head bowed, wearing a swimsuit next to a pool.

The 20-year-old's announcement came without any notice from Swimming Australia. The team's assistant coach Michael Bohl confirmed she would not swim at Gwangju but said the team had no idea why.

"We are not exactly sure. She left the team which we are obviously very disappointed about but she has left for personal reasons," Bohl told Australian media.

"We all respect the privacy she has asked for and we will find out in time. There has been absolutely no indication (of her reasons). We were just told at a team meeting that she has had to go.

"We respected that but we are moving ahead positively. Sometimes you lose people along the way - hopefully we find out in the next two, three, four weeks."

Although Jack helped Australia claim the relay world record with the Bronte sisters and Emma McKeon at the Gold Coast Commonwealth Games last year, Bohl said her place at Gwangju was never guaranteed, with Madison Wilson and Brianna Throssell providing competition.

"There is nothing saying Shayna was the person going in," he said. "Shayna wasn't in a position where she was 100 percent guaranteed a finals swim — she had to get into the heats and prove herself." — Reuters