The whale was seen last month in waters near the beachside suburb of Maroubra in New South Wales.
The NSW National Parks and Wildlife Service (NPWS) confirmed the “extremely rare” sighting on Friday.
Blue whales – the largest animals on Earth – are rarely seen so close to the shore, it said.
Mr Marshall said the sea creature may have been more than 25m (82ft) in length and weighed more than 100 tonnes (100,000kg).
Yet despite their size, blue whales are “largely ‘invisible’ even to the most avid whale watchers”, Mr Marshall said.
“They are not often seen because they tend to live very far out to sea, their populations are widely dispersed and we have very limited data on its migration and critical habitat,” he said.
The rarity of the sighting was not lost on one photographer, who managed to take pictures of the whale as it swam along the coast near Maroubra.