A customer who purchased chicken breasts from a Coles supermarket in Victoria claimed they "won't be shopping there again" after discovering green flesh inside the fillet.

Jamie Ferguson bought the RSPCA approved chicken from Northcote Place in Victoria's inner north. She posted a picture on social media with the caption: "Careful folks. This is the chicken they're selling at Coles. Won't be shopping there again."

But it turns out the curious colour change is actually quite normal, albeit undetectable before the bird is cut open, news.com.au reports.

A spokeswoman for Coles told news.com.au they had contacted the customer and offered a full refund or replacement. She said the green flesh is known as deep pectoral myopathy (DPM) and is usually the result of a "bird flapping its wings too much".

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"DMP is a green discoloration of the flesh caused when swelling occurs as a result of oxygen deficiency in the muscle," she said.

"This usually happens from the bird flapping its wings too much. It is not harmful to eat and the taste of the chicken would not be altered."

She said the quality of the supermarket's products is "very important" and encouraged anyone who encountered an item they were not 100 per cent happy with the return it.

It's not the first time a chicken from Coles was cut open and dinner immediately ruined. In 2016, social media blew up over claims a chicken "appears to have gangrene". A Coles spokesman reassured those concerned that it was DPM.

Researchers have noted that cases of DPM are increasing but "the lesion does not impair the general health of birds and is generally found during cut-up and deboning".