California Evidence Code Section 664 Interpretation
According to Evidence Code section 664, there is a rebuttable presumption that an official duty has been regularly performed.
For example, once the DMV presents competent evidence in the form of documents contemplated in the statutory scheme to establish its prima facie case, the licensee must produce competent affirmative evidence of the nonexistence of the presumed facts sufficient to shift the burden of proof back to the DMV. (Cf. Jackson v. Department of Motor Vehicles (1994) 22 Cal. App. 4th 730, 739 [27 Cal. Rptr. 2d 712].)
The licensee must show, "through cross-examination of the officer or by the introduction of affirmative evidence, that official standards were in any respect not observed . . . ." (Davenport v. Department of Motor Vehicles (1992) 6 Cal. App. 4th 133, 144 [7 Cal. Rptr. 2d 818].)
Once such showing has been made, the burden shifts to the DMV to prove that the test was reliable despite the violation. (Ibid.)