A hole "almost the size of a football field" has opened up at Jumpinpin bar on North Stradbroke Island, off south-east Queensland.
Senior lifeguard Michael Bates said the beach had collapsed on the southern side of the island this morning.
It is a popular spot for fishing and four-wheel-driving but no-one was on the beach at the time.
Mr Bates said the erosion had created dangerous conditions.
There is still a question mark over how big the hole will get.
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