In the clip, recorded by 65-year-old Fethi Ada, you see a dark-looking object appearing to float along the river.
It dunks its head under the surface before appearing again.
The video then cuts before we can get a closer look at what this could possibly be.
The footage was recorded during a family fishing trip on the Hawkesbury River in Australia.
“It looked human the way it lifted its head up”
Speaking to 7News, the man who recorded the clip said: “It looked human the way it lifted its head up.
“It looked over towards us like who are these people.
“My brother threw some bait in and then it just got scared off.”
Rob Jones, 35, of North Wales filme this video
But, Secretary of Hawkesbury River Rescue, Felicity Brownrigg, viewed the video and was not as convinced about what it was.
Felicity said: “It’s rare you actually get to see many (animals) as they stay below the surface.
“My guess would be that it is a seal.”
Seals are not commonly known to reside in this part of Australian waters.
Felicity also said the occasional bull shark and whale calf have been spotted in the river.
Just last week terrifying footage was captured of a man surfing over a bull shark.