Tau, 31, who has four Test appearances, joined the Basque side last October after serving a two-year ban for a masking product used to cover up performance-enhancing drugs.
According to local newspaper Sud Ouest, Tau was involved in an incident whilst driving a vehicle given to him by the Top 14 side.
The forward, who had signed a new deal in March keeping him with the outfit until 2022, crashed into two other cars but no-one was hurt.
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