The 27-year-old, who played for Australia at schoolboy and Under-20 level, tested positive for benzoylecgonine, the main metabolite of cocaine. According to Pau, he "admitted that he had taken the drug in a private context" and "without any member of the club being present".
Johannesburg-born Hewat, who joined Pau in 2021, has only played six matches this season after being injured at the start of the season. His last match was against Castres on January 4.
Pau stressed the "immense disappointment and anger generated by this news" and said that the player "has already been given a provisional suspension".
The club did not rule out further sanctions but said that Hewat "will be subject to an appropriate medical support programme".
Pau, 11th in the Top 14, play host to Wales' Ospreys on Saturday in a bid to secure qualification for the Challenge Cup finals.