Captain Frederick van Wyk said about 120 workers at Sandrivier estate prevented other workers from entering the premises and burned the vehicle and a piece of field on the farm.
No orchards were damaged.
He said police were deployed to prevent more damage and the workers were escorted to an open field.
Twelve people were arrested on charges of public violence and malicious damage to property.
No injuries were reported and no shots were fired.
Agri Wes-Cape spokeswoman Porchia Adams said the workers were seasonal and had been taking part in an unprotected strike on the farms of the JC Le Roux group, in Paarl, Wellington and Piketberg.