Five-hundred cows stolen from a mid-Canterbury dairy farm were probably rustled over a period of months, police believe.

Police continue to investigate the "complex" theft from the Ashburton farm property, which was reported last month.

The cows could be worth as much as $1m.

Senior Sergeant Scott Banfield said investigators have "spoken to and interviewed numerous persons" that may or may not be connected.


"Our inquiry has established that although not impossible, it is highly unlikely that a large number of cattle have been removed over a short period of time," he said.

"If any offence has taken place, it would have been progressed gradually over months."

Banfield said it's a timely reminder to farming communities that security on farms must be a priority.

"A lot of reported thefts are committed by opportunist criminals simply looking for that insecure shed or farm vehicle with the keys left in the ignition," Banfield said.

"When employing farm staff, farms should carry out due diligence around any applicants to minimise the risk to their business and to their livelihood."

Any information relating to the rustling should be forwarded in confidence to the Ashburton Police 03 307 8424.

Alternatively, information may be provided anonymously to Crimestoppers on 0800 555 111.