Doubts about the egg industry's ability to police its producers have been raised after a Sun-Herald analysis found the total of free-range layer hens in the country were incapable of producing the total of free-range eggs sold each year, and as many as one in six eggs labelled free range on retail shelves were cage or barn-laid.

Despite providing annual figures on the number of free-range eggs sold, the Australian Egg Corporation has admitted it has no way of knowing how many free-range layer hens exist.

No government agency collects such data, but the Bureau of Statistics' Agricultural Commodities noted the number of chickens for egg production fell 3 per cent in 2007-08. At the same time the Egg Corporation was reporting a 48.2 per cent rise in sales of eggs labelled free range.

The NSW Greens pledged yesterday to introduce a bill to create a rigorous definition of free-range eggs and push for an egg labelling accreditation scheme.

The president of the Free Range Egg and Poultry Association of Australia, Ivy Inwood, said any producer could accredit themselves as free range, and the Egg Corporation's criteria for voluntary accreditation was enabling large operators to cash in on consumer demand, producing eggs under conditions most people would consider barn-laid.

The Egg Corporation said it would support any investigation of egg substitution. Its website says it uses third-party auditors for its accreditation program.