Cryptozoologist Rex Gilroy said he recently saw the 12m giant surface in the Hawkesbury River. Through binoculars Mr Gilroy saw a dark shadow ``with a longish neck'' near Wiseman's Ferry.
Mr Gilroy, who has been searching for Sydney's beast since 1965, said he believed it was a plesiosaur from the Jurassic period. Mr Gilroy, known for his research on the western Sydney panther, said neither the NSW nor Scottish Nessies were alone.
"We'd have to have a breeding population of no less than 300 to 600," he said. "We're dealing with ocean creatures coming into the river to breed. There are areas of ocean ... anything could live down there and you wouldn't know it.''
After hearing of the Hawkesbury Monster in 1965 he found accounts dating back to pre-colonial times, with stories told of children being attacked by the ``moolyewonk''.
When fishing boats were found overturned and the occupants missing in the 1980s, the Hawkesbury Monster was the prime suspect. "We have rock art depicting them. It seems the Aboriginal people knew of these creatures," Mr Gilroy said