Australia's Apostle family gets bigger

The limestone stacks are a major tourst attraction along the Great Ocean Road (AAP)
The limestone stacks are a major tourst attraction along the Great Ocean Road (AAP)

Australia's famous Twelve Apostles have stood proudly off the coast of Victoria for eons, but researchers have just discovered what lies beneath the surface -- five more long-lost relatives.

Sonar mapping has for the first time uncovered the five 'Drowned Apostles', found 6km offshore from the Great Ocean Road and 50m below the surface.

Australia's Apostle family gets bigger

An impression of the five newly discovered limestone stacks (Rhiannon Bezore)

PhD student Rhiannon Bezore, Associate Professor David Kennedy from the University of Melbourne's School of Geography and Deakin University's Dr Daniel Ierodiaconou made the discovery, which was presented at the International Coastal Symposium in Sydney today.

The five new additions are smaller in size than their cousins, but the fact they exist at all has surprised them because of their "defiance of normal erosion rates".

It's understood this is the first time such limestone stacks have been found preserved in the ocean.

"Sea stacks are always eroding, as we saw with the one that collapsed in 2005, so it is hugely surprising that any could be preserved at that depth of water," Assoc Prof Kennedy says.

"They should have collapsed and eroded as the sea level rose."

Like the Twelve Apostles, the Drowned Apostles would have once been part of a larger limestone sea cliff and date back around 60,000 years.

Australia's Apostle family gets bigger

Researchers say the 'Drowned Apostles' should have eroded ages ago (Liz Rogers)

Ms Bezore, who made the initial find in sonar data, initially couldn't believe what she was seeing.

"We had to check what we were seeing because no one has seen stacks submerged at this sea level before," she says.

Their existence comes about as a result of a finely tuned set of circumstances -- the rock needs to be soft enough to erode quickly from a cliff, but hard enough to support a rocky pillar.

The data was part of a larger project to map the reef in Victoria, which supports commercial fisheries for southern rock lobster and abalone.