Adrian Chew and his partner Stephanie purchased the 500 gram jar of Remano Tomato Paste from the Aldi store at Abbotsford in Melbourne's east last week.

They opened the jar on Wednesday night, they said, only to find it inhabited by the squirming, white larva.

"We were going to make spaghetti bolognaise, but it didn't eventuate," Mr Chew said.

"The jar made a popping noise when we opened it, so it hadn't been opened before."

The pair believe the maggot was clinging to the underside of the lid of the jar, before they opened it.

"Stephanie was mortified to say the least," Mr Chew said.

The label on the jar of tomato paste reads: "Especially made for our Aldi stores to our stringent quality specifications."

It is listed as a product of China.

Aldi responded to the couple's complaints on Thursday evening, apologising to them for their experience and pledging to investigate how the maggot-laden tomato paste got on the shelf.

"Our customer service team contacted the customers today to advise that their concern had been received and will be swiftly addressed. We've contacted our supplier to make sure this concern is thoroughly investigated and will implement any further action required," an Aldi spokesperson said.

"Aldi Australia meets all statutory requirements regarding Food Safety and undertakesregular testing by an independent laboratory. We work closely with our suppliers to ensure they meet our strict specifications for quality, taste, texture, nutritional content and appearance."

Garry McDonald, an entomologist from Melbourne University, said it was highly unusual for a maggot to be found living in a jar containing a processed food product like tomato paste.

"It's a pretty acidic, hostile environment," Dr McDonald said.

"The average life of a maggot - from egg hatch to developing into a pupa - might take two or three weeks. It could be a bit less, or a bit more. It's not all that long.

"If the maggot was alive, the egg would have had to have been laid in the last two or three weeks."

Dr McDonald said flies typically laid their eggs on food, rather than on a container.

"In a case of a tightly sealed jar, it's also implausible that a fly of some sort would lay eggs on the outside, and that the egg would hatch and that the young maggot would find its way inside," he said.

The couple still have the jar with the maggot inside and are willing to take it to Melbourne's Agribiosciences Centre for testing.

The pair has contacted Aldi via Facebook and Twitter and filled out a form documenting the incident, as requested by the supermarket.

Mr Chew said he and his partner had only recently started shopping at Aldi.

"There's definitely a cost saving there, but this has soured the experience a little bit I think," he said.

His partner Stephanie said she wanted to warn other shoppers who may have purchased the same brand of tomato paste to be vigilant and check the jar before using the product.

"If I can spare someone biting into a maggot then my disgusting discovery will be worth it," she said.

With Alana Schetzer