The RSPCA is investigating poultry farms in a crackdown on illegal cages, reports Australian Broadcasting Corporation.

Last year, all egg producers were required to increase space for hens. This generally meant increasing cage sizes by the diameter of a beer coaster.

The RSPCA's Melina Tensen says almost 20 per cent of farmers have not complied.

"It's close to two million hens that are currently in cages that are not complying with new regulations, so that's a rather large number of hens," she told ABC.

RSPCA's 'Choose Wisely' Campaign

The RSPCA says its growing list of Choose Wisely restaurants is increasing demand for barn-laid and free-range eggs, with 435,456 eggs every year no longer being sourced from caged hens.

"Ultimately the RSPCA would like to see all hens living a cage-free existence but this is a great start," said RSPCA Australia Scientific Officer, Melina Tensen.

"In less than four months, we've signed-up ten businesses throughout the ACT and Western Australia, all committing to use barn-laid or free-range eggs as much as they can in their restaurants and cafés.

"We're hoping a new advertising campaign [launched on 15 January], will help drive recruitment in other states" The RSPCA's new ads can be viewed by clicking here.

"Unable to express natural behaviours as simple as stretching out and flapping their wings, battery hens suffer chronically throughout their short lives.

"The positive news is that this is one animal welfare issue where every decision you make, whether you’re a business or a consumer, can make a difference. In fact the purchase of just six barn-laid or free-range eggs a week can help one hen live a cage-free life," said Ms Tensen.