"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.