The consumer group Choice says half the extra virgin olive oils it tested failed to meet international standards.

Choice got the NSW Government's Olive Oil Testing Service to examine a single bottle of 28 popular brands taken from supermarket shelves, and found 14 did not meet the International Olive Council (IOC) standards for extra virgin oil.

Many of the oils were also given low scores by an expert tasting panel.

The best tasting olive oils were almost universally Australian made, with nine out of the top 10 tasting brands produced locally.

Choice says the best buys were the Coles Australian, Cobram Estate and Woolworths Select Australian extra virgin olive oils, which met the IOC standards, performed highly in the taste test, and were less expensive than some other local brands.

Choice says the better performance of local oils is largely because olive oil deteriorates with storage, transport and the amount of time after harvest before it is used.

"Extra virgin olive oil deteriorates with time and exposure to excess heat, oxygen and light," Choice's report noted.

It says some producers and retailers have responded to its report in an effort to improve freshness and quality.

"FoodWorks and Aldi are taking steps to rectify problems along the supply chain, while Ollo and Red Island have strengthened internal production processes to avoid quality issues," it wrote.

"But distributors and producers of other oils that failed our tests have not indicated they will take any action."

Choice says the results demonstrate the need for official testing to ensure that oils labelled extra virgin comply with minimum international standards.

"With the absence of any mandatory standard in Australia you have to trust that oil labelled "extra virgin" is the real deal," said CHOICE spokesman Brad Schmitt in a statement.

"This means it's been mechanically extracted from quality olives without the use of heat or chemical processes and has low acidity. Unfortunately this isn't always the case."

The full choice report is available here.