According to a statement on the official website of Swimming Australia on Saturday, the test was conducted by the Australian Sports Anti-Doping Authority (ASADA) on June 26, and the swimmer Shayna Jack was "provisionally" suspended from the Australian Swim Team and went back to Australia from a training camp being held in Japan.
Jack took the gold last year in the Gold Coast Commonwealth Games as part of the world record-breaking 4x100 women's freestyle team, reports Xinhua news agency.
Local media reported that the 20-year-old swimmer "mysteriously withdrew from the world championships" earlier this month citing "personal reasons."
"Officials initially failed to elaborate on the circumstances behind her exit," said The Australian. But the incident was revealed by a whistle-blowing local media on Saturday, which said that the cause of the swimmer's withdrawal was due to "an abnormal A sample during routine, pre-competition testing."
Swimming Australia has been criticized for its handling of the Jack case, mainly because it did not address the issue when it was revealed on Saturday - instead leaving it to Cate Campbell, a teammate of Jack's.
"I do want to say that while an Australian athlete returning an adverse result is both bitterly disappointing and embarrassing to our team, our sport and our country, it does not in any way change the zero tolerance view that Swimming Australia has and our continuing fight for a clean sport," Swimming Australia chief executive Leigh Russell told reporters on Sunday.
"I do accept the criticism that we did not have an official speak poolside last night and that Cate Campbell spoke on behalf of our team. That was my call.
"In retrospect we could have done that differently but I do want to acknowledge Cate and her leadership and our team."
It comes days after Australian swimmer Mack Horton made a spectacle at the championships in Gwangju, South Korea, by snubbing Chinese gold-medalist Sun Yang on the podium.
Speaking on Australian Broadcasting Corporation (ABC) television on Sunday, Richard Ings - the former head of ASADA - accused Swimming Australia of trying to cover-up the case.
"The public do notice, and ultimately what was said by Shayna Jack and Swimming Australia weeks ago about vague personal reasons become transparent weeks later as a lie. The truth needs to be told at the beginning," Ings said.
( With inputs from IANS )