Federated Farmers board member and rural policing spokesperson Richard MacIntyre told RNZ's Afternoons with Jesse the case was "very distressing".
"Farmers will be very concerned to know that stuff can happen like this on this sort of a scale."
He estimated it would have taken two or three trucks to move 500 lambs and said the theft would have required a lot of planning.
"It would've been quite a logistical exercise to organise the people, the dogs, the trucks, to get them through a yard and onto the trucks for example, or to ... potentially drover them down a road to get them to wherever they were going," McIntyre said.
"It certainly couldn't just have been pulled off by one person."
While livestock thefts on such a large scale were "uncommon", McIntyre said rustling was a problem for New Zealand farmers and most of the thefts went unresolved.
He said in some cases livestock was stolen to order by other farmers, but the "vast majority" was taken to be processed and then sold as sheep or beef.
"There are people that steal for other farmers; they steal cattle and sheep with a view to taking them to someone else's farm to be finished before going to the works," he said.
"I guess like every part of society ... most people are upstanding but you've got the odd individual that is a bit less so."
However, he believed "farmer-to-farmer thefts" were not the main issue: "The vast majority are small numbers that go essentially into the food chain."
Identifying stolen stock was theoretically possible as lambs had ear markings specific to the farm they were born on, and cattle had electronic ear tags, McIntyre said.
But cattle's ear tags could be removed or replaced and "obviously if they're in the form of steak or roast lamb it's pretty hard to track them down".
McIntyre said it was prohibitively expensive to insure livestock and it could be difficult for farmers to actively monitor every part of their farms to prevent thefts.
"We're dealing with hundreds of hectares of land and if you think of motion detection cameras ... they've only got a detection range of 20-odd metres at the most, so you've got to be pretty lucky to have them in the right place that people will access your farm through."
Despite that, farmers should set up surveillance systems around their properties' main access points and try to keep valuable stock as hidden as possible, he said.
"Keep anything that you think people might be wanting to steal - for example, prime lamb and beef - away from the road so people can't see it or easily access it."
Police said they received a report on Sunday that 800 lambs were missing from a property in Hurunui District.
They provided security advice to prevent further livestock loss and were making enquiries, they said in a statement.
- RNZ
