Willie Rioli stands accused of one of the most serious offences under the World Anti-Doping Code; deliberately tampering with a urine sample.

Unless he can convince Australian Sports Anti-Doping Authority investigators or, more likely, an AFL tribunal, that he didn't do it, he faces a potentially career-ending, four-year ban.

Willie Rioli has been provisionally suspended by the AFL. Credit:AAP

The accusation against him is that he substituted his urine sample, an example of what WADA classifies as chemical and physical manipulation.

"If it is a substitution there is a degree of premeditation," says former anti-doping chief Richard Ings.