Police say local knowledge would have been needed for a major sheep rustling operation in the Wirrulla district on South Australis's west coast.

They say 100 pregnant merino ewes have been stolen from a property, with the total value estimated to be $10,000.

Ceduna CIB officer Dave Sheldon says it was a substantial and well-organised heist.

"There would have to be some level of organisation but it's difficult at this stage to say whether it's a group of people or whether it's a small operation in terms of the number of people running it," he said.

"Speaking to people in the community there are thoughts that if you had a good sheepdog or two you could probably achieve something like this with very few people involved."