i'd work on a litre or more per person - and have LOTS of water available (not sure of the quality of Sydney water, but we make sure to have bottled water when we have drinkies) - if you have people drinking, it's good to have water for them over night or early morning (or if like me, i drink a good half litre before sleep to flush out the hangover). for those that aren't used to lots of sugary drinks, they might have a glass of softdrink and then a drink of water in between...