A lab test found seven out of 12 popular oregano products were found to contain less than 50 per cent oregano leaves. They were bulked out with olive and sumac leaves.

The worst offender was Master of Spices, which was only 10 per cent oregano, followed by Hoyt's, at 11 per cent, and Aldi's Stonemill, at 26 per cent.

Aldi has pulled the product from shelves, and has begun offering full refunds.

"It is unclear why the adulteration occurred, but the fact is you shouldn't require a lab test to tell you what herb you're actually buying. You should be able to trust a product labelled 'oregano', is actually oregano," said spokesman Tom Godfrey.

"There is clearly a major problem in the oregano supply chain in Australia, which also raises the question about other herbs and spices entering the Australian market."

The test results also showed Spice & Co and Menora's products, were only a third oregano, Spencers was 40 per cent and G Fresh was 50 per cent. The packaging all declared the product was of a single ingredient: oregano.

In good news, MasterFoods, Woolworths Select, Coles and Herbies Spices products were found to contain 100 per cent dried oregano leaves.

The oregano test was conducted following an investigation by UK consumer group Which? last year that found a quarter of 78 samples were contaminated with other ingredients.

Its results showed olive leaves and myrtle leaves could constitute 30 to 70 per cent of the final product.

"Clearly we have identified a major problem and it may well reflect issues with other herbs and spices that enter the British Isles through complex supply chains," Professor Chris Elliott, director of the Institute for Global Food Security, said at the time. "Much better controls are needed to protect the consumer."

Food fraud continues to be an ongoing concern for authorities around the world. Cumin has been recalled multiple times in the United States because it was found to be cut with undeclared peanut - a fatal allergen for some.

In 2013, European consumers were shocked by the discovery of horsemeat in frozen beefburgers. More recently, various brands of coconut milk have been removed from Australian shelves because of undeclared cow's milk.

Choice has referred its findings to the Australian Competition and Consumer Commission, saying the adulterated products are likely to have breached Australian Consumer Law.

"We're pretty certain the suppliers of the seven products didn't know their product was tainted, so there's no suggestions from us that they knowingly deceived consumers," said Mr Godfrey.

Australians splurged $115 million on herbs and spices last year, according to the Retail World Annual Report. Oregano has found new favour in Australian kitchens, with the growing popularity of mediterranean cooking.

The 12 products were purchased from supermarkets, grocers and delis in Sydney, Melbourne and Perth.

The labelling on the Master of Spices product says, "Ingredients: Oregano rubbed" and "May contain traces of nuts, gluten and sesame seeds". While it declares "Packed in Australia" and "Proudly Australian owned", there is no indication about which country the herb was sourced from.

The Hoyt's product, sold at Woolworths and Coles, claims on the label: "All natural" and "finest quality". It also says "packed and processed in Australian from imported product".

Both Master of Spices and Hoyt's did not respond to requests for comment, but Hoyt's did tell Choice it was co-operating with the ACCC investigation.

And Aldi spokeswoman said the company recently became aware that between January 2015 and March this year the Stonemill product did not meet its product specifications and subsequently pulled it from shelves.

"We have given customers the opportunity to receive a full refund by returning the product to an Aldi store by 31 May, 2016," she said.

"We are working with our suppliers to ensure that this product contains 100 per cent oregano before it is back on shelves."