SYDNEY - James Magnussen and his teammates from the Australian men's 4x100m freestyle relay squad have admitted using a sedative banned by their national Olympic committee in a bonding session before the London Games.

The prescription drug Stilnox was banned by the Australian Olympic Committee (AOC) just before the 2012 Games and the athletes now face sanctions from the governing body for breaching their Olympic team membership agreement.

Stilnox, a brand of the medication zolpidem, is not banned by the World Anti-Doping Agency but was prohibited by the AOC because of its hallucinatory effects and after former Olympic champion Grant Hackett said he had become addicted to it.

Members of Australia's 4x100m freestyle relay team from the London Olympic Games raise their hands as they sit with Daniel Kowalski (4th L), CEO of Australian Swimmers Association, and Barclay Nettlefold (4th R), president of Swimming Australia, during a media conference at a hotel in Sydney, Feb 22, 2013. [Photo/Agencies]

"We stand here collectively to confirm that we did take part in a bonding exercise during which members of the relay team took Stilnox," the six swimmers said in a statement read out at a news conference in Sydney on Friday.

"We acknowledge by our recent action in continuing a recent tradition in the Australia swim team, we have let ourselves down and the people who have supported us."

Olympic silver medallist Magnussen and his team mates Matt Targett, Eamon Sullivan, James Roberts, Cameron McEvoy and Tommaso D'Orsogna apologised for their actions.

The statement said the drug had been prescribed before the AOC ban and the swimmers had then indulged in "childish" and "stupid" pranks such as knocking on the doors of other athletes.

The AOC said they would be appointing a lawyer to conduct an investigation into these and other disciplinary issues relating to the swimming team in London.

"All members of our 2012 team were well aware of the ban imposed. The AOC will consider what action it will take in the light of these admissions," read a statement.

"Possible sanctions for a breach of the team agreement might include withdrawing funding from the athletes concerned."