A former farm employee is "pivotal" to their investigation into the theft of 500 cows from an Ashburton farm, police say.

Senior Sergeant Scott Banfield told Fairfax police inquiries centred around a staff member at Norvo farm "and his activities".

Banfield did not name the man.

In a statement police added their investigation had indicated "auditing systems and processes may have been open to abuse" and that they had spoken to several people about the theft.

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They also warned farmers to ensure they completed background and reference checks before hiring.

Earlier this month nzfarmer.co.nz estimated Norvo owners would be out of pocket by around $2 million because of the theft.

Authorities urged anyone who may have information in relation to the case to come forward.

**Can you help? Call Ashburton Police on (03) 307 8424 or CrimeStoppers anonymous on 0800 555 111.