There are two broad categories of drink driving offences in New South Wales: driving with a prescribed concentration of alcohol (ie novice, special, low, middle or high range p.c.a.) and driving under the influence (or D.U.I.).
The most frequently charged offence is P.C.A., which is where you are tested at the roadside and later at the police station and return a blood alcohol concentration above that which is allowed by the law.
By contrast, driving under the influence or D.U.I. is a charge brought when police have not, for whatever reason, been able to lawfully test your blood alcohol concentration. A D.U.I. is normally more difficult for the police to establish than a P.C.A. because they must prove beyond a reasonable doubt you were actually affected by alcohol or another drug at the time of driving in the absence of a blood alcohol reading. To do this, police will normally rely on factors such as your manner of driving, your demeanour and anything else which may suggest you were affected at the time of driving.