A giant, fossilized tooth of an ancient killer whale was discovered in Beaumaris Bay.

The huge tooth measuring 30 centimeters (12 inches) long, estimated to be about 5 million years old from the Pliocene epoch, is a proof that killer sperm whales once inhabited Australian seas.

Murray Orr, a fossil enthusiast was walking along the beach of Beaumaris Bay - a known fossil site, when he stumbled upon the giant tooth.

"After I found the tooth I just sat down and stared at it in disbelief," said Orr. "I knew this was an important find that needed to be shared with everyone."

Murray donated the fossil to Museum Victoria.

The teeth belonging to the bay sea monster found in Beaumaris! @7NewsMelbourne 4pm pic.twitter.com/rIISvQMwAU

— Jacqueline Felgate (@Jacquifelgate) Abril 21, 2016

Dr. Erich Fitzgerald, Senior Curator of Vertebrate Paleontology at Museum Victoria said the fossil would be used for scientific research and education.

"If we only had today's sperm whales to go on, we could not predict that just five million years ago, there were giant predatory sperm whales with immense teeth that hunted other whales," said Fitzgerald.

Fitzgerald said that the dental dimensions were bigger than that of a Tyrannosaurus rex. He also noted that the tooth looks incomplete - the tip of the crown and some base of the root are missing and could be from a whale which was not fully grown yet.

Fitzgerald is intrigued as to how only one of the species of the whales continues to survive today. Through the fossil, he wants to know how the whales co-existed in the past, how the oceans supported them, and why they were lost.

The fossil belonged to an extinct sperm whale species, which is believed to be related to Livyatan melvillei that lived during the Serravallian stage of the Miocene epoch, about 12 to 13 million years ago.

Fitzgerald said the finding is valuable in preserving Australia's fossil heritage.

Just recently Australian scientists completed years of fossil analysis, which revealed a new dinosaur species - Kunbarrasaurus.

Photo: Amila Tennakoon | Flickr