This mystery woman, the one with the Australian accent, probably was not Australian at all. She probably came from anywhere but Australia. She probably approached the random British tourist and said to him "the dingo ate my baby" in a mock Australian accent. People do that sort of thing all the time, just like Elaine did in that Seinfeld episode at the party in Long Island.
The English tourist, being English and a member of the banking community, had no sense of humour but a head full of tabloid stories. So he thought he heard an Australian saying something about a baby arriving, and everyone took him at his word. Now Australia is full of English private detectives trying to find a glamourous Australian with a baby-butch haircut. Their considerable expenses are being paid by all those folk who made donations to the McCann fund or who bought the t-shirt. People say the funniest things, don't they?