This rock formation on Victoria's south coast is known as London Arch.
It used to be called London Bridge, for its resemblance to that other well-known landmark, until 15 January, 1990, when the larger of the two rock arches collapsed into the sea, leaving the gap you see here between the remaining arch and the mainland.
Up until that time, there was a tourist walkway out on to the formation, with a lookout at the far right end of the arch in this picture. And yes, people were on it at the time of the collapse. Thankfully nobody was on the bit that fell into the sea, but two people were stranded on the newly formed island until rescued by helicopter some hours later.
So it's true... these sorts of coastal formations do collapse unpredictably.
Port Campbell National Park, Great Ocean Road, Victoria.
See where this picture was taken. [?]