See Stephen Mark Wright v David Ronald Morton [1997] VSC 46. The facts were the appellant was the driver of a motor vehicle that had been involved in an accident.  He had an accident, and his car hit a tree. He admitted drinking alcohol with his passengers sometime before the accident, and shortly after the accident whilst waiting for the ambulance to arrive. 

He was taken to hospital, and a blood sample taken from him at hospital at 9:25pm.  The estimate as to when the accident had occurred was 6:20pm.  The Magistrate concluded that although he could not be satisfied beyond reasonable doubt that the blood sample was taken within 3 hours, he could apply the presumption of continuance to approximate the accused's blood alcohol concentration when he was driving the vehicle.  The Magistrate convicted the defendant on this basis.  He appealled on a question of law to a single Judge of the Supreme Court and was unsuccessful.  He further appealled to the full court of the Supreme Court.  

On appeal the Court  of Appeal held that the Magistrate had erred.  The Court acknowledged other scenarios where it was possible for a Court to infer from surrounding circumstances, and after hearing expert evidence what the approximate BAC level of a driver may have been at the point of driving. 

it was in the Court's opinion speculative on the facts before it, to attempt to gauge what the defendant's BAC may have been at the point he drove the car.   The court observed the complexities involved in a back-calculation outside the 3 hour period, taking into account the accused's sex, weight, build, the rate at which alcohol was absorbed and eliminated.  The taking of blood 3 hours within driving was deemed critical by the Court.