Section 47K of the Road Traffic Act 1961 (SA) states that where the police have complied with the requirements relating to breath analysing instruments and procedures, then the concentration of alcohol indicated by the breath analysis instrument as being present in the blood, is presumed to have been present in the blood at the time of analysis and throughout the preceding 3 hours. Prior to 12 December 2022 this time period was 2 hours.

If the breath analysis can be said to have been done more than 3 hours after the driving or attempted driving, then the police cannot rely on the presumption [see for example, Moore v Police (1997) 27 MVR 116; [1997] SASC 6448]).

If a driver wishes to challenge the accuracy of the reading of an alcotest or breath analysis machine taken within 3 hours of their driving or attempted driving, they will have to undergo a blood test. If a blood test is not taken, the result of the breath test cannot be challenged other than in exceptional circumstances (see consuming alcohol after driving).

A blood test will usually provide a reading lower than that provided by the breath analysis. This is to be expected because it is highly unlikely that the breath analysis and the blood test will be performed within the same hour. Usually a matter of hours will pass before the driver is able to get to a hospital and have a blood sample taken. During this time their blood alcohol level will naturally start to fall, provided they have not been drinking in the interim.

When challenging the results of a breath analysis it is necessary to determine the alcohol elimination rate of the person concerned. The alcohol elimination rate measures the rate at which an individual eliminates alcohol from their blood. Alcohol elimination rates vary from person to person but generally between 0.01 to 0.02 gm of alcohol per 100 ml of blood per hour is considered within the normal range, in addition to a 25% margin of error.

In order to establish that a breath analysis reading was inaccurate, the results of the blood test must be explained and interpreted to the court by a medical expert who can provide evidence about the driver’s alcohol elimination rate at the time of the offence [Tonkin v Police [2006] SASC 145]. This would require further testing through a laboratory at the driver’s expense.