Aus hospo workers: your advice / opinion on people who BYO food into restaurants.

I'm a couple of years into working at a restaurant in a tourist region in Tasmania. We have an occasional issue with people arriving with food they have purchased elsewhere and sitting at our inside and outside tables to consume it. We do make an exception for individuals with serious food allergies who have arrived as part of a group and informed us before being seated - generally though it will be a group that orders a couple of coffees and then proceeds to unpack their sandwiches, chips, biscuits etc before being politely told that we do not allow outside food to be consumed at out tables.

What is the best way to explain to people that they cannot BYO food? Apart from a flat 'we are a private business and not your public picnic table, mate'. Are there health and safety regulations that I can cite in order to shut down any arguments they might try? Busy season is coming and I really CBF with the time wasted on telling people over and over that they have to put that tub of manky Coles salad that probably been sitting in their hot car for an hour back into their bag

EDIT: thank you for the advice from other hospo workers - we have decided to go with a straight up 'our restaurant policy is that we do not permit outside food to be eaten at our tables'. If anyone pushes back they will be informed that they will find that most restaurants typically do not allow BYO food and drink on the premises. No need for getting into the why's and why nots with the salad roll smugglers. I appreciate the feedback and advice I have gotten here. Also, those of you who strongly believe that buying a cup of coffee entitles you to take up an entire table inside a restaurant for an hour or two while you eat your servo fish and chips and your 2 hour old KFC before walking out and leaving your rubbish behind....lol WTF?! Would you bring your 4 kids to a cinema and argue with the proprietor because you've bought 1 ticket it shouldn't be an issue if the kids grab 4 free seats if no-one else is sitting there and it's not costing anyone anything? I bet you wouldn't.