Revelations surfaced on April 17 that a joint investigation - involving multiple government agencies, including NSW Police - was under way following reports horses had been butchered at the property and their carcasses left in a dry creek bed.

The exact location of the property - from which authorities believe an illegal knackery was operating - has not been revealed but the council's general manager, Peter Thompson, said it could not be seen from surrounding public roads.

The alarm was raised after the owner of an adjacent property spotted horse carcasses at the location.

"What was alleged to us was that the carcasses were being deposited into a water course and that then gave us a power of entry to go onto that property to investigate," Mr Thompson said.

The information received led the general manager and other council staff to enter the property, alongside representatives from the police, the NSW Food Authority, Local Land Services, the Department of Primary Industries and Racing NSW.

"Initially the focus of the investigation was to confirm whether the carcasses had been left in the dry creek bed and possible offences under the Protection of the Environment Operations Act," Mr Thompson said.

"Once the inspection of the property commenced it became clear that the slaughtering of horses had been occurring for a long period of time."

The horses, which Mr Thompson said have not been identified as racehorses, are believed to have been shot dead and dumped over several years.

Numerous separate dumps of carcasses were discovered at locations throughout the property.

"It is estimated that there are in excess of 500 horse carcasses. Some of these carcasses were no more than skeletal remains while others were killed relatively recently," Mr Thompson said.

"Once the extent of the operation had been identified, NSW Police and other state government agencies began collecting evidence for possible offences and regulatory actions under a range of NSW state government legislation."

No branding was identified on the horses and some live horses were also found.

"It's our understanding it's an illegal knackery that was operating," Mr Thompson said.

"This would have been an activity where horses were brought to the property for this purpose."

Mr Thompson said he had contacted the property's owner to let them know an investigation would be taking place and there was subsequently an attempt to conceal some of the carcasses with soil before authorities arrived at the location.

He said identifying those responsible for the knackery was complex because it was believed multiple people were involved.

The general manager also revealed the initial information was received about eight weeks ago and the probe was being kept under wraps to ensure its integrity.

However, details about the investigation were made public after one of the city's councillors revealed information provided during a confidential briefing.

"The reason the story has emerged is that we are about to commence regulatory action," Mr Thompson said.

"I provided an update to councillors on Monday night because it's a significant issue, not just for Wagga but much further afield.

"I thought the councillors needed to be aware of what was about to unfold."

Mr Thompson said he didn't know which councillor released the information or why.

"It's disappointing," he said.

No water contamination has been identified and Mr Thompson said there was no concern of contamination.

However, he said the council was focusing on the surrounding land and would be responsible for the disposal of the carcasses.

Mr Thompson said it was not uncommon for horse meat to be used for pet food but that would form part of the NSW Food Authority and DPI's investigation.

The council said the matter remains a current investigation.

A NSW Environment Protection Authority (EPA) spokesperson said the council was the appropriate regulatory authority to investigate potential pollution and waste offences in relation to the matter

"The EPA has assisted Wagga City Council with advice regarding the issue of alleged slaughtered horse carcasses at a premises in Wagga," the spokesperson said.

"The EPA will continue to provide assistance to the council and other agencies as needed."

The NSW DPI has been contacted for comment.