A horrified Sydney shopper had the fright of their life after finding a live snake in a bag of cos lettuce from Aldi.

The Mosman woman claims her son found a "baby pale-headed snake" in the packaged salad item, sharing two photos of the dangerous reptile slithering among the green leaves.

She explained her shocked son was forced to contact Wildlife Information, Rescue and Education Service (Wires) which came and collected the misplaced snake.

Aldi Australia said it was investigating how the snake found its way into the product.

A close-up of the live snake inside the bag. Photo / Facebook

"We've worked with the customer and the team at Wires to identify the snake's natural habitat, which is certainly not an Aldi store," an Aldi spokesperson said.

"We thank Wires for their support on this. We are working closely with our produce supplier to investigate how this incident could have possibly occurred."

The woman wrote on Facebook yesterday: "Check packaged lettuces carefully. Last night my son found a baby pale headed snake in with his baby cos lettuce from Aldi."

Her post was met with shock from other social media users, with one calling it "bizarre" as others pushed for more details.

"Was it still alive?" one asked.

"That is bizarre. What did he do next?" another said.

While others called it an "awful" and "nasty" find.

The mum responded, revealing it was "very much alive" but thankfully her son was okay, though he "hadn't realised how dangerous it was".

On the son's own Facebook page, the reaction was just as strong, with one person saying the find made them feel "physically sick".

Others saw the funny side, making jokes that the snake had given them a good excuse never to eat salad again.

"Like I needed another reason to loathe salad," one wrote.

"Creating a fear of vegetables," another agreed.

A pale-headed snake is a "potentially dangerous species" according to Queensland Museum, which states its venom is haemotoxic.

The effects of a bite by a pale-headed snake can be "serious" as the venom destroys red blood cells and disrupts blood clotting which can cause organ failure.

It can be found in central eastern Australia, from Mareeba in northeastern Queensland down to mid-eastern New South Wales.