A reminder for farmers to keep a closer eye on stock during winter, as the mysterious case of 500 missing cattle remains unsolved.

Police say the Ashburton farm's auditing systems and processes may have been open to abuse.

Federated Farmers Mid Canterbury President Michael Salvesen said electronic tags on dairy cows monitor production and stock numbers in modern milking sheds daily.

But during winter, the animal's aren't milked or brought inside.

"If you don't count them every day or even every month, then in that time you leave open the possibility for something to go wrong on a bigger scale."