Activists dressed in chicken suits blocked the entrance to the mill for several hours, holding flags demanding "Feed me GM-free" and turning distribution trucks away.

Other campaigners scaled the silo and unfurled a banner reading "INGHAM - putting GM in chicken".

The objective was to raise public awareness that Ingham was the largest user of GM crops in Australia.

It was widely unknown that Ingham chicken feed contained GM soy as there was no requirement for any reference to the fact to be made on product labels said Greenpeace spokeswoman Helen Oakey.

"Greenpeace is taking this action on behalf of Australian consumers who don't want genetically modified crops anywhere in the food chain," said Ms Oakey.

"As Australia's leading chicken producer Ingham needs to follow the lead of the dairy industry and publicly commit to using non-GM soy to feed its stock.

"There is plenty of non-GM soy available on the world market and with an annual turnover of about $1.2 billion Ingham is the 14th largest private company in Australia and it certainly has the capacity to go GM-free.

"Making the change would cost less than one cent per chicken - it's chickenfeed."

Similar action was undertaken at the Ingham silo at Newcastle while a delegation led by Greenpeace chief executive Peter Mullins took the GM-free message to the company's corporate headquarters at Liverpool.

A statement from Inghams Enterprises issued in response to the protest said its products conformed to all government regulations and argued that the use of GM soy did not compromise their absolute GM-free status.

"There is no GM DNA evident in the animal tissues," it said.

The company - whose two mills supply approximately 7000 tonnes of feed weekly - also expressed concern for the welfare of chickens affected by deliveries being prevented.