Family films 'strange' creature in Hawkesbury River - but can you tell what it really is?

A Sydney family fishing on the Hawkesbury River were stunned by the sight of a large creature splashing near their boat.

Fethi Ada was wetting a line with his siblings, daughter and grand-children near Patonga when the creature broke through the water’s surface.

“It looked human the way it lifted its head up,” Mr Ada told Yahoo7 News.

“It was very strange, we saw it for about 10 seconds."

The Greystanes kebab shop owner said he quickly pulled his leg inside the boat when the animal began flapping around.

“It looked over towards us like who are these people,” the 65-year-old added.

“My brother threw some bait in and then it just got scared off.”

Secretary of Hawkesbury River Rescue, Felicity Brownrigg, viewed the video to identify the marine creature.

“It’s rare you actually get to see many (animals) as they stay below the surface,” she told Yahoo7.

“My guess would be that it is a seal,” she said.

She also said the occasional bull shark and whale calf have been known to make their way into the river.