A Sydney couple have been left shocked after opening a tin of baby formula they had bought at Big W to find it full of dirt.

Stuart Stirling purchased the tin of S-26 brand progress baby formula at the retail chain’s Narellan store in south west Sydney last week.

Mr Stirling told nine.com.au his wife then opened the tin earlier this week.

“I was actually in my office at home when my wife opened it. She was in the kitchen. And I could hear her saying, ‘what the hell, what the hell’,” he said.

“When I saw it I couldn’t believe it.”

As well as the dirt, the formula tin also contained chicken pellets, Mr Stirling said.

A picture showing the contents of the formula tin, which Mr Stirling posted on Facebook, has been shared almost 500 times in two days.

Commenting on Mr Stirling’s photo, Victorian mum Kelly Higgins said a similar situation had happened to her last week.

“Same thing happened to me last week except it was half formula and I think half sand,” Ms Higgins wrote.

Ms Higgins said she had bought the formula from a store in Geelong, but it is not known if it was a Big W store.

Mr Stirling said the store had offered to exchange the tin for a new one, but he would not be doing so “out of principle”.

Mr Stirling said he was prompted to share his photo to raise awareness about Big W’s policy of allowing refunds for baby formula tins.

Mr Stirling said he believed someone had bought the tin of formula and used it, before filling it up with dirt to get a refund.

“We have got some baby scales at home and I put the tin on the scales. It was a couple of grams off (the formula tin correct weight) and it was only three quarters full.

“So obviously dirt is heavier than formula so someone worked out if I fill it three-quarters full it’s going to feel the same.”

“The fact that it was $20 doesn’t bother us. To be honest there have been a few sceptical comments on that post, a few non-believers, and due to that I’m not going to exchange it, out of principle.

“I just wanted to raise awareness and let parents know to check the seal.”

A spokesperson for Big W said they were investigating the Mr Stirling's case as a matter of priority but it appeared to be an isolated incident.