Stuff reports that even SPCA-approved free range eggs are under the spotlight after the animal welfare agency found some of its accredited free range egg suppliers were not meeting standards.

The revelation follows a Newsroom investigation that revealed millions of caged eggs were being sold at New Zealand supermarkets as free range.

The SPCA is also now fast-tracking plans to introduce traceability for its Blue Tick products - which consumers turned to after Newsroom revealed free range egg brand Palace Poultry was buying caged eggs to meet a shortfall in free range supply.

While the Serious Fraud Office investigates Palace Poultry, Newsroom's investigation has thrown the poultry industry into disarray and consumer confidence into doubt.

Now it can be revealed that the SPCA issued notices to "a small number" of its accredited Blue Tick members.

The Blue Tick scheme sets higher standards than set out in New Zealand law for free range egg farming. Independent AsureQuality auditors conduct annual and random visits to Blue Tick members to ensure they are meeting benchmarks.

SPCA New Zealand acting chief executive Andrea Mudgen said the issues raised with its members since this week's controversy were now resolved.

"All of these issues have been addressed according to our strict and timely processes.

"However if there was ever a critical issue then we would not hesitate to immediately put the farm on hold, and they would no longer be part of the SPCA Blue Tick scheme. We would also immediately contact the distributor to ask them to stop buying from that farm."

In the wake of the Newsroom investigation into Palace Poultry brand eggs, Countdown has pulled the brand from its shelves and three other brands - Woodlands, Farmer Brown and Select - have been implicated as possibly selling caged eggs as free range in the past.

Countdown has also called in AsureQuality to audit all its suppliers of free range labelled eggs.

The Blue Tick system is considered one of the most reliable by consumers, who have been left doubting the ethical claims on the more expensive eggs.

Ms Mudgen said discussions began last year about introducing traceability - an idea also being proposed by the Poultry Industry Association (PIANZ).

Those plans were now being "fast tracked" to give consumers "a final guarantee to ensure their [eggs'] origin is accurate", she said.

Consumer New Zealand's senior research writer Jessica Wilson said the best way for consumers to ensure their eggs met their welfare standard was to do their own research.

On its website, the Blue Tick approved free range egg brands are listed as Henergy Cage Free, The Natural Free Range Company, Higgins Free Range, Uncaged Hens, Pams Cage Free, Bon Appetit, The Ultimate Egg Company's Free Range and Barn, Sungold's Organic and Cage Free, Golden Downs, Doug's Free Range, Simply Cage Free and Ewing Poultry.

The Blue Tick Scheme

• Has higher standards than those written in NZ law for welfare of free range hens
• Follows the principles of hens to be free from discomfort, injury, hunger and distress and have the freedom to express natural behaviour
• Uses independent auditors AsureQuality to do annual and random checks on its brands