Free-range egg farmers have backed calls for an independent accreditation system, which they say is urgently needed to protect the sector's integrity.

Australia's Farm Online reports that 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 Ltd (AECL) 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 three 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 New South Wales Green Party has pledged 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 that any producer could accredit themselves as free-range, and the Egg Corporation's criteria for voluntary accreditation enable large operators to cash in on consumer demand, producing eggs under conditions most people would consider barn-laid.

The Egg Corporation told Farm Online it would support any investigation of egg substitution. Its web site says it uses third-party auditors for its accreditation programme.