A Sydney woman wanted to know what was hanging in her neighbour's trees. Photo / Supplied

A Sydney mum was confronted with a scene out of a science-fiction movie when she spotted a collection of mysterious "pods" hanging from a tree in her neighbour's backyard.

The woman posted to a northern beaches Facebook group, asking for help identifying the large sacs, as she was worried they looked rather "sinister".

"At the risk of looking like I have no idea (well I don't), anyone know what these dangling things might be from the tree next door?" the woman wrote.

She also posted a series of photographs of the unidentified sacs to help illustrate her point.

According to the woman, the pods appeared to have "spider webs around them".

"Wondering if it's something a little sinister," she wrote.

"It's hanging from next door's tree close to our trampoline so (I) was a little worried!"

Locals jumped to help identify the pods, explaining they were the work of magnificent spiders, or bolas spiders.

One woman said the spider species was "super cool".

"When the eggs hatch, the baby spiders parachute down with little parachute webs!" she wrote.

The mum didn't like the thought of that though.

"Oh god, all I can see are parachuting spiders gliding down onto the trampoline now!" she wrote.

According to the Australian Plants Society NSW, magnificent spiders "pose no threat to humans".

They also explain that the "spindle-shaped egg sacs" are constructed over several nights and contain about 600 eggs. As many as seven sacs can be attached to a branch.

Baby spiders emerge from the egg sacs in late winter and early spring and are "carried away by the wind".

One woman on the Facebook group likened the hatchlings parachuting to the story of Charlotte's Web.

"My fav story as a kid! Her babies all hatched and parachuted down, I always thought that was so cool!" she wrote.

But not everyone on the Facebook group was a fan of the spider sacs or their escape method.

"Millions of parachuting little spiders … hmm, can they be re-homed in the bush?" one woman asked.

Another woman warned the mum to "leave the country now … and don't look back", while another posted a photograph of a can of Mortein spider spray, claiming several hundred eggs, multiplied by four egg sacs, was "too many spiders" for her to handle.