Anyone remember the Tourism Australia campaign from way back where Paul (Crocodile Dundee) Hogan said he'd put "another shrimp on the barbie" for you? Aussies would never use the word shrimp: here they are prawns, thankyouverymuch. But they do love their barbies, and what you'll find most often on an Aussie barbie is a sausage (also known as a snag or at least 3 other slang terms that my children know better than I do).
As far as I know, the Sausage Sizzle is a uniquely Australian phenomenon. They are fundraisers, a kind of bake-sale-meets-barbecue event. We've seen them raising money for everything from childcare centers to cancer foundations. On election days, there are even Sausage Sizzles at polling stations (finding a captive audience for fundraising: Aussies are required to vote).
On weekends, pop-up barbecue stalls at the local equivalent of Home Depot grill sausages, onions, and serve them on a slice of bread. Tomato sauce (ketchup) and H&P sauce (which isn't exactly steak sauce or barbecue sauce but a little bit of each) are avaialble to squeeze on top.
Like most fast greasy food, it smells incredible. There is huge expectation for the first bite (you probably went out erranding on little more than a cup of coffee). And, like most fast greasy food, your gut tells you to regret the entire event soon after the last bite is finished.