It can depend on where the car is parked.
For example, if it's parked in a place where consuming alcohol is prohibited*,
then a person would be consuming alcohol in that place.**.
That they are in sitting in a car, as opposed to, say,
sitting in the open at a picnic table, would not be a factor.
In Queensland, there is also a Liquor Act offence for consuming alcohol in the car park of licensed premises***
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* often, but not always, under Council by-laws, but also under Part 6A of the Liquor Act 1992 (Qld);
** which might be, say, a park, a car park of an oval, or at a beachfront,
but probably not private land;
*** which is lawyer-speak for the car park of a pub or a bottle shop or similar (and yes,
that can include the car park of a shopping centre where a bottlo is one of several shops);