The rider selected in front of Armidale's Rowan Willis in the Olympic showjumping team has tested positive to cocaine.
Jamie Kermond, who was named in the starting side for the Tokyo Games, has been provisionally suspended.
Equestrian Australia released a statement on Wednesday.
"The mandatory Provisional Suspension Notice asserts Mr Kermond returned a positive A-sample for a metabolite of cocaine following a test conducted by Sport Integrity Australia on 26 June 2021," they said.
"Cocaine is prohibited In-Competition by the World Anti-Doping Agency (WADA) and the Australian National Anti-Doping Policy 2021.
"Mr Kermond is prohibited from participating in any WADA compliant event, including the Tokyo 2020 Olympic Games, while the provisional suspension is in place."
Kermond has the option of having his B-sample tested.
Previously listed as a reserve on the team, Willis has confirmed "it's too late" for a call-up.