Police are investigating the disappearance of up to 500 dairy cows from a farm in mid-Canterbury.

Farmer Pennie Ormsby-Saunders took to Facebook in an attempt to track down the missing herd.

"We have had approximately 500 cows stolen from our large dairy farm in the Ashburton area," Ms Ormsby-Saunders wrote.

"If anyone has seen or heard anything suspicious we would love to hear from you. You could also contact the Ashburton police."

Ms Ormsby-Saunders told Newshub she has a herd of 1300 cows but last week noticed more than a third of them were missing.

The cows were in calf and grazing over winter south of Ashburton.

Each cow would be worth more than $1000.

Newshub.