The Surf Scoter is rarely seen outside North America but has been spotted on Manly Dam reservoir.
The black-feathered bird has an oddly-shaped, multi-coloured bill and usually lives along the coast.
Wayne Ball, a fellow duck lover, said: "It's put [the reservoir] on the map as a site for such birds."
"It's very nice to have the bird sighting here which is the first sighting in Sydney.
"Only four have been spotted inland in the state , and there's been less than 20 records in the country," he said.
The Surf Scoter is also known as the Goggle-nose or Horse-head Coot and has a pronounced hump on the top of its bill.
Hundreds of thousands of the birds spend the winter in the coastal waters off British Columbia in Canada.