Part of one of Australia's most famous natural landmarks has been reduced to a pile of rubble after one of the ruggedly beautiful Twelve Apostles rock pillars collapsed in front of stunned onlookers.

A 45-metre pillar that geologists say took 20 million years to form took just seconds to collapse into the Southern Ocean on Sunday after witnesses said a large crack appeared in its side.

"Reports were it sort of shimmied and shuddered a bit, it fractured and sort of imploded in on itself and pretty much slid straight into the ocean," Victoria state park ranger Alex Green told Australian Broadcasting Corporation radio.

"So we've gone from a 50-metre stack to a 10-metre pile of rubble," he said.

Mr Green said the pillar's collapse was part of the natural process of erosion that had formed the rugged coastline along Victoria's Great Ocean Road, one of Australia's most popular tourist spots about 220 kilometres southwest of Melbourne.

Before Sunday's collapse, there had been nine of the imposing rock formations still standing just offshore.

The pillars were formed over millions of years as relentless wind and waves eroded soft limestone cliffs, forming caves which then became arches as the limestone collapsed and then stacks isolated from the shore.

Part of another of the stacks, known as London Bridge, collapsed in 1990.