The theft of 500 dairy cows from a Canterbury property would have needed about 13 trucks, a Federated Farmers' leader says.

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The cows worth about $800,000 vanished from an Ashburton property managed by Mark and Pennie Saunders.

Police received a report of the theft several days ago and yesterday Pennie Saunders appealed for information on Facebook.

"Recently we have had approx. 500 cows stolen from our large dairy farm in the Ashburton area," she said in the post. "If anyone has seen or heard anything suspicious we would love to hear from you. You could also contact the Ashburton Police."

Federated Farmers mid-Canterbury former president Willy Leferink said the cows, which were about to calve, disappeared from a herd of 1300.

He said it would take up to 13 truck and trailer loads to remove that many cows from a farm - and it was not possible to walk a herd that big on Ashburton roads without someone noticing.

Mr Leferink said it was not clear how the theft was carried out.

The Saunders were distraught, he said.

"When you have that many cows missing a cold chill will go through you and then you'll say 'it can't be true'.

"And where do you go with 500 cows?"

Mr Leferink said the cows were each worth between $1400 and $1600 and he understood they were not insured.

He said insuring dairy cows was extremely expensive.

Ashburton Police said they had visited the property, had taken statements and were continuing their inquires.

Senior Sergeant Scott Banfield said farmers needed to ensure they were taking measures to help prevent stock thefts.

"All farmers should be checking their boundaries and fence lines on a regular basis.

"It's unlikely the theft of hundreds of animals could be completed at once, and is more likely that multiple thefts could be carried out over a period of time.

"Regardless of the size of your property, ideally stock numbers should be checked once a week.

"Farmers should also be keeping an eye out for unexpected signs of herding near their boundary lines too."

Mr Banfield said farmers should also review their auditing systems.

"There are almost always improvements which can be made to the management and security of stock. Clearly tag animals and keep accurate stock numbers.

"Enhanced security and signage can help to deter opportunistic thefts."

Federated Farmers' South Canterbury dairy chairperson Ryan O'Sullivan said it would take a very skilled person to carry out such a theft but it was not impossible.

The theft of more than 100 cattle from Canterbury farms last December remains unresolved.