A number of items were seized following searches at two properties in Forkhill during a multi-agency investigation to target agricultural crime on Monday.
The investigation was also aimed at protecting the integrity of the food chain and the Food Standards Agency (FSA) is currently investigating which businesses could have been supplied by the producer.
Ulster Unionist MLA Danny Kennedy said the incident had caused serious widespread concern that cattle rustlers could be involved.
"Rogue operations within the agricultural industry have the potential to damage the food chain," he said.
Mr Kennedy said there is a "great suspicion" that organised criminality was behind the underground operations.
The Newry and Armagh representative added: "It remains to be seen what the police take will be on this, but it's important that they establish it as quickly as possible and give clarity.
Over the last three years, some 9,000 cattle have been 'rustled' or stolen all through Northern Ireland and there are areas of concern such as Co Armagh generally and in south Armagh particularly.
Danny Kennedy, UUP
He added: "The border areas seem to be worst hit, and that implies rogue operators and an element of criminality that has to be dealt with."
Barclay Bell, deputy president of the Ulster Farmers' Union said he didn't believe what had been uncovered was common practice.
"When you look at the overall picture, there will be a rogue element within any industry, no matter what it is," he said.
"Hopefully over the next few days we will see all the agencies involved getting to the root of the trouble and stopping this sort of practice."
Mr Bell said considering the traceability system in Northern Irish farming, it would be difficult for legally traced animals to end up in an illegal slaughterhouse or meat plant.
"I think there has to be concern that some of these animals may well be stolen animals that have been disappearing off our farms," he commented.
"There have been a quite a large animals that have disappeared and really as yet we have seen no convictions.
"We certainly would like to see the agencies getting to the root of this trouble and I think it would be a big relief to all of our farmers."
Even if product has entered the food chain, effective cooking would minimise the risk to public health.
Health Minister Edwin Poots
The farming spokesperson wouldn't speculate if meat from the illegal site had ended up in local shops or butchers.
He added:"What we can say is that we have a very robust farm quality assurance scheme within Northern Ireland, I think we can reassure [people] that if they're buying their meat from a reliable source, I don't think they've anything to fear."
Mr Bell said animals cannot legally be killed on a farm, even for personal use, and must be taken to a recognised abattoir.
DUP Health Minister Edwin Poots told the assembly on Tuesday that the FSA are monitoring any possible risk to public health.
"Full risk management procedures will be implemented by the FSA if investigations reveal that products from this premises entered the food chain," he said.
"The main risk arising from this type of operation is microbiological contamination of product. In the event of contaminated product entering the food chain, aside from removing it, the main way to deal with bacteria is to cook the meat well."
But SDLP DARD Spokesperson and Vice-Chair of the Agriculture Committee, Joe Byrne has expressed his concern regarding the ministerial handling of the issue.
"Give the serious issue that this could have been for public health and the serious impact on the reputation of the beef industry, I am very concerned that the DARD Minister and the Health Minister have not met formally to deal with this issue at the highest Ministerial level," the West Tyrone MLA said.
"I feel strongly that the DARD Minister Michelle O'Neill should have come to the Assembly today and outlined what measures DARD are taking to deal with this issue with the urgency it deserves and what they are doing to restore and maintain public confidence in the beef industry.
"It is very disappointing that a first-hand account was not given by the DARD Minister in the Assembly. At the very least the Minister should have spoken to the Agriculture Committee to reassure us, the farming community and the wider public."
Both the PSNI and the Food Standards Agency said it was too early in their investigations to say if stolen cattle had been used in the suspected illegal food processing plant.