Around 500 dairy cows worth around a million dollars have gone missing from a farm in Ashburton.

Farmer Pennie Ormsby-Saunders has posted a message on Facebook to try to track down the missing herd.

The cows were in calf and would be worth more than 2000 dollars each.

Ms Saunders said she knows three of four other farms in the area that have been affected by livestock theft.

Canterbury Police are asking anyone with information which could be useful to contact them.