An anonymous whistleblower told The Daily Advertiser this week that the tragedy occurred when an air-cooling system failed at the Pig Improvement Company's piggery in Grong Grong on the weekend of February 21 and 22.

A security company was alerted when the system failed but was unable to reach on-call employees to notify them.

Pig Improvement Company chief executive Paul O'Leary said staff were deeply affected by the incident.

"The welfare of our animals is our highest priority at all times," Mr O'Leary said.

"This is why we have 24 hour monitored alarm systems which trigger when power supply is disrupted.

"Despite these precautions, in this instance the alert was unable to avert a disaster – a situation every business dreads.

"The consequence was distressed staff arriving on site the following morning to discover the stock losses.

"These animals are their livelihoods and they care for them every day. Losses like this cut deep emotionally for all staff."

He said a subsequent on-farm veterinary investigation revealed the deaths were caused by a lack of ventilation.

The company is reviewing the integrity of its warning system and has vowed to make changes if necessary.

In response, animal protection group Voiceless said: "This represents a disgusting and unacceptable failure to ensure animal welfare, and one which would have caused those 500 sentient beings unimaginable suffering.

"This horrific report comes at a time when big agribusiness and federal senator Chris Back are calling for reforms that would reduce public scrutiny over animal industries. That these animals reportedly died in such a horrifying way, and the public and relevant authorities have apparently been kept in the dark for weeks, raises serious concerns around the transparency and accountability of this industry.

"Now that this information has finally come to light, important questions must be asked: How can the Australian public possibly have faith that animal industries have appropriate systems in place to ensure animal welfare on factory farms? And how can we ever be sure that welfare breaches are reported when employees within these industries are so unashamedly silenced?

"Voiceless calls for the relevant authorities to conduct an immediate investigation as to whether there has been any violations of the Prevention of Cruelty to Animals Act 1979 (NSW)."

Humane Society International Australia has called for an amendment to the Animal Welfare Code of Practice for Commercial Pig Production to specify that all intensive establishments that have 200 or more confined animals must have at least one staff member on site 24 hours a day.

The code currently specifies that, for pig housing which is electronically ventilated, in the event of a power failure or malfunction, the producer must ensure there is an alternative power source, or the shed must be opened up to allow natural ventilation.

"Given that they are part of the world's largest and most extensive pig breeding and technology company, surely PIC Australia can afford the extra staffing to monitor their intensive piggeries 24 hours a day, avoiding the despicable suffering that 500 breeding sows endured last month," Verna Simpson, director of the society, said.

"These poor creatures were confined with nowhere to go, and would have experienced a long and painful end.

"If simple measures had been taken then this terrible incident could have been completely avoided.  HSI has written to the Minister for Agriculture, Barnaby Joyce, to strongly urge him to amend the code to prevent a repeat of this welfare disaster in the future," she said.

In May last year, animal activists released video footage and images from inside the Grong Grong Piggery, allegedly depicting piglets in pieces and hundreds of pregnant sows confined to stalls.

The Daily Advertiser contacted the piggery for comment but was directed to Australian Pork Limited (APL).

General manager of communications at APL, Emily Mackintosh, said the footage was staged and the dead piglets had been scattered across the ground in an act of "sensationalism".

Ms Mackintosh said the vision was obtained illegally during a break-in and stressed the high biosecurity risk such an act posed to the industry.

The piggery was established in 1974 and has 25 employees.

with Andrew Pearson and Steve Jacobs