Centrelink says it is cracking down on people who are receiving benefits and simultaneously making large profits on the trading website eBay.

The agency has asked the website to reveal the identities of about 15,000 people who have sold more than $20,000 worth of goods in one year.

The head of Centrelink, Hank Jongen, says many people may have declared the money legally, and he says the agency is not targeting those who use the website as a hobby.

But he says the Government wants to make sure the system is not being abused.

"The aim of this program which is really a pilot, is to identify people who may be receiving Centrelink payments who are operating an organised business in the online cash economy in order to hide profits," he said.

"The emergence of new technology has meant that in reality we've got to become more innovative in the way we detect fraud.

"Online trading creates an opportunity for people to earn income and in some instances we believe people may not be declaring that income."

Mr Jongen says eBay is co-operating with the Government.

But the president of the Australian Council for Civil Liberties, Terry O'Gorman, says it is a gross invasion of privacy.

"What we say should happen is that if police have probable cause for investigating someone, they go to a magistrate, they get a warrant and they access that person's eBay records that way," he said.