Ribbonwood Station's manager, who referred media inquiries to Federated Farmers, said it was a major financial blow.
"On August 26, we undertook a stock count, but by September 15, when we'd brought our sheep in for pre-lamb shearing, we'd lost 200 ewes.
"Speaking to neighbouring farmers, it seems around 600 stock have been stolen over the past three years.
"That's concerning," he said.
Mr Rose, who is finalising a letter of agreement with police aimed at having better communication between the rural community and police, described the theft as "just wacko".
He urged anyone with possible information about the theft to contact police.
"Every little bit will help build the picture that will help apprehend them," he said.
The resources needed to steal 200 ewes indicated a "degree of sophistication", Federated Farmers meat and fibre chairwoman Jeanette Maxwell said.
"The number of ewes stolen needed a stock truck, musterers and dogs.
"You are talking about a sizeable and sophisticated operation.
"Stock are clearly being stolen for a particular market given they're in-lamb ewes.
I would remind farmers to only use reputable suppliers when purchasing stock," Mrs Maxwell said.
She asked people to be extra vigilant "because these stock will have been moved somewhere".
"If merino ewes appear on a block, as if from nowhere, it may well be worth calling your local police.
"If stock are legitimate, there will be a paper trail.
"We'd also ask farmers to keep an eye out for unknown stock agents offering capital stock."
She urged farmers to work with their neighbours and record licence plate numbers or take photographs of people or vehicles acting suspiciously.
Any thefts of stock needed to be reported to police.