The AFL confirmed on Tuesday that Rioli tested positive for a metabolite of cannabis in a September 5 ASADA drug test after West Coast's elimination final win over Essendon.

Cannabis is an in-competition prohibited substance under the Australian Football Anti-Doping Code, meaning the breach could attract a ban of several years.

It's not yet clear whether the maximum ban is two or four years for a positive cannabis result.

Rioli is already facing a ban of up to four years after allegedly tampering with his urine sample during an ASADA test on August 20.

It's been reported that on that occasion, a frustrated and dehydrated Rioli poured a sports drink into his sample jar after being unable to pass urine for more than three hours.

But his positive drug test for cannabis will raise questions about Rioli's actual motives during the alleged urine tampering.

Former ASADA chief Richard Ings said that because the positive drug test for cannabis happened on September 5 - before Rioli was notified on September 11 of his alleged urine substitution - it is considered a concurrent rather than consecutive matter.

Therefore, according to Ings, Rioli is facing just the one penalty - whichever one is worse.

Ings said that a positive test to cannabis might only attract a suspension of a few months.

He also had this "free tip" for athletes.

"If you have to choose between an ADRV (anti-doping rule violation) allegation for substitution or an ADRV allegation for cannabis, choose cannabis," Ings tweeted.

"The sanction for cannabis is much less and cannabis is only tested for on match day."

Ings said that if a player allegedly attempted to substitute a urine sample, then it's guaranteed they would be target tested very soon afterwards.

Rioli recently posted a picture of himself and West Coast development coach Chance Bateman together in the Northern Territory.

West Coast said they weren't in a position to go into detail about Rioli's positive drug test.

"As stipulated two weeks ago, the club's primary concern remains the health and wellbeing of Willie," the Eagles said in a statement.

"We will continue to support him through this particularly challenging time to assist him while this ongoing investigation runs its course.

"We expect that these processes will take some months to complete and the club will not be in a position to make further comment until such completion."

Rioli is provisionally suspended while the matter plays out.

Australian Associated Press