Mystery over the case of 36 dairy cows missing from an Ashburton farm.

The disappearance was discovered by farm staff when they noticed milk production was down.

Owner Jill Quigley said she's tried ringing every transport operator in case they know something, but has had no luck.

Quigley said with no sign of the cows having been walked off the farm either, their removal remains a complete mystery.

Senior Constable Mike Seque said a milking shed tracking system has helped police narrow down exactly when the cows were taken.

"On the 9th there were 20 cows that didn't come into the shed, and then again on the 7th and the 8th 16 cows didn't come into the shed."

Seque is urging anyone who has seen something suspicious in the Valetta area to contact them.

"We can speculate all day about what may or may not have occurred, but even if they can get a registration number that's fantastic. It gives us a line of enquiry, if they've seen any vehicles that may have been capable of moving that many cows we'd love to hear from them."

The 36 cows are worth close to $70,000.