When I played on a rugby team with a bunch of Aussies that was one of rugby party challenges - mostly for Americans (That ‘Men at Work’ song was popular then!). A drink of beer, vegemite on a cracker, drink of beer again vegemite on a cracker, repeat until you threw up or ran out of beer. I have no idea where they got it, I’ve never seen it for sale here (Though I haven’t really looked!) I assume they brought it in or their mommas sent it to them.