Nathan Baggaley, who won two silver medals at the 2004 Summer Games in Athens, and his brother had pleaded not guilty.
But a Brisbane jury found both men guilty following a two-week trial, Nine News reports.
CNN has reached out to the crown prosecutor Ben Power and representatives to the brothers for comment.
Dru Baggaley and another man were arrested in the summer of 2018 after law enforcement intercepted their boat, according to Nine News.
Authorities seized more than 500 kilograms of cocaine, according to court documents.
Court documents detail the prosecution’s evidence linking Nathan Baggaley to the case, noting that his fingerprint was found on a piece of tape used to cover the boat’s registration plate.
He also attempted to contact his brother Dru while he was at sea, according to court documents.
The boat had met a larger foreign vessel off the coast of New South Wales and was subsequently intercepted, according to the prosecution’s case as outlined in court documents.
The brothers have been convicted and served time in connection with drugs charges in the past, according to court documents and local media reports.
They will now remain in custody until sentencing and possibly face life imprisonment, per Nine News.