A rarely spotted blue whale measuring about 30 metres and weighing almost 200 tonnes made a quick stop in waters off Sydney, Australia.

Blue whales are believed to be the largest animals in the world and are rarely seen, according to local media.

"I've been whale-watching here in Sydney for five years and I've never seen one," Jonas Liebschner of Whalewatching Sydney told Australian TV channel Network Nine.

About five thousand of these giant mammals exist worldwide, local media reported.

"It is very, very rare to see a blue whale off Sydney. I think the last confirmed sighting is over 20 years ago," Liebschner said.

Tourists on a whale-watching boat were entertained on a sunny spring day as the whale made a shortcut on its long journey to Antarctica.

Experts say krill attracted the giant to Sydney's waters which is rarely spotted so close to the coast.