500 in-calf dairy cows have been reported stolen on New Zealand's south island, with the owners holding little hope of getting them back.

Workers on the dairy farm near, the town of Ashburton, discovered the cows were missing as the herd was being gathered as they neared their calving dates, Stuff New Zealand has reported.

Pennie Saunders, an administrator for the farm, told Stuff that they noticed last week that "things weren't adding up as cows were coming back in".

Cows were spread around the farm over winter and brought back in as they neared calving, it was only when workers started counting the cattle that they noticed the missing herd.

"We're absolutely gutted, it's just disbelief," Saunders said, with their dairy farm usually having a herd close to 1,300 cattle. 

Saunders said that the chances of retrieving the animals was slim and that there had been several cattle thefts in the area.

He was hesitant to give specific details of the mass cattle theft to Stuff New Zealand "while investigations are ongoing".

We're not very hopeful of getting them back. Chances are their tags have been removed, which makes them pretty much impossible to trace.

"They could have gone to the works (sent to slaughter), or could be on someone else's land I guess. People do it for the money," Saunders.

In a statement, police said they were advised of the missing cows on Thursday and were "making enquiries".