Ex-swimmer Brenton Rickard has pleaded innocence at a doping hearing that could strip Australia of Olympic medals for the first time.

Rickard and his lawyers appeared via teleconference before the Court of Arbitration for Sport (CAS) on Monday night (AEST) after retrospective testing showed a diuretic in his sample from the 2012 London Olympics.

Now aged 37, Rickard swam a breaststroke leg in a heat of the 4×1000-metre medley at the London Games. He was replaced for the final, in which Australia won bronze.

Rickard, James Magnussen, Christian Sprenger, Hayden Stoeckel, Matt Targett and Tommaso D’Orsogna could lose their medals from the event at the London Games.

An Australian athlete has never been stripped of an Olympic medal because of a positive drugs test.

The CAS hearing was confidential, but Rickard’s legal team had previously flagged the furosemide in Rickard’s sample as a known contaminant of an over-the-counter medication.

But his lawyer Rebekah Giles, who has been contacted for comment, had acknowledged difficulty in proving innocence eight years after the event.

CAS, based in Lausanne, Switzerland, released a statement following the hearing the confidential nature of evidence.

CAS said it will deliver findings “if any sanctions are imposed, once the award is final and binding”.

“In this instance, the parties agreed during the hearing to issuing a short statement,” a CAS statement.

“Going forward … the case will remain confidential.”

Rickard late last week sent an email to swim colleagues about his August 1, 2012 test, saying the re-test found “an exceedingly small concentration” of furosemide.

“The re-analysis did not detect the presence of any other prohibited substance that may have a performance-enhancing effect,” he wrote, adding the positive test was his “worst nightmare”.

“I have always abhorred doping within the sport so you can imagine how sickened and horrified I am to find myself in this predicament,” he wrote.

Rickard also raced at the London Games in the 100-metre breaststroke and 200-metre breaststroke, finishing sixth and seventh in the respective finals.

A former president of the Australian Swimmers Association, he also won silver medals at the 2008 Beijing Olympics in the 200-metre breaststroke and 4×100-metre medley and is a three-time Commonwealth Games gold medallist.

-AAP