Nearly $30,000 worth of stock has been rustled from a South Otago farm, in what Southern police are calling the biggest rustling in some time.

Senior Sergeant Trevor Thomson, of Dunedin, said an Akatore farmer reported 170 lambs and 30 ewes missing from his property yesterday.

The sheep were taken over a three month period after the farmer's last head count at the property east of Millburn, he said.

The ewes were worth $110 while the lambs would earn the rustler $150 each, taking the total value to $28,800.

Police had not encountered such a big rustling in a long while, Snr Sgt Thomson said.

''It's a lot of stock to be missing.

''It's one of the larger ones.''