"Mazda's conduct towards these consumers was not just appalling customer service as noted by the judge, it was a serious breach of the law," ACCC Chair Rod Sims said in a statement.
Mazda Australia said it was carefully considering the federal court finding, but declined to comment further.
The court, however, dismissed the regulator's allegations that Mazda engaged in "unconscionable conduct" in its dealings with these customers. It will decide on penalties and other orders sought by the ACCC at a later date.