Surfers and beachgoers at a northern Sydney suburb, Dee Why's beach, were left stunned at the sight of a number of green, sponge-like spheres, which washed ashore.

RaeMaree Hutton, Dee Why Surf Life Saving Club's patrol member told The Manly Daily:

"I didn't want to touch one because you never know what can sting you on the beach, but I did poke it with my toes and it's squishy, like a sponge. They look like alien eggs or something."

Local resident, Jenny Zhang, further added: "About three days ago, there were a few egg-shaped balls but then today, they were much bigger and everywhere on the beach.

While locals in Dee Why explored extra-terrestrial origins, scientists clarify that the sponge-like spheres are in fact seaweed that cluster together in the form of an egg to protect themselves from predators.

Alistair Poore, Associate Professor from the School of Biological, Earth and Environmental Sciences at the University of New South Wales in Sydney says the balls could be a rare type of living green algae.

"I've seen similar things - sometimes dead sea grass can roll around and form balls like underwater tumbleweeds but that's made of dead material and these look to be living. It is a habit known as "aegagropilious", where the algae is free living (not on rocks) and forms into spherical balls."

Last month, bioluminescent creatures lit up Sydney's Manly Beach's waves giving a rare sight to photographers and Newport residents.