If what glenb says is correct (I'm sure he's right but don't put money on CASA
) I would have thought it was okay as long as you were not taking part in safety sensitive activities. If your duty hours were over (as per the sign on sheets or whatever your company procedure is) and you weren't doing anything safety sensitive it would seem to be permissable.
Personally, however, I will be retiring to the pub for any after work drinkies, because then there is no ambiguity. If staying on premises, maybe a change of clothes would be advisable.