"We might not hear of it up front or it being reported as a crime. Farmers don't muster their stock every day, so they might not know if a few sheep have gone missing, or they might think they have just escaped to a neighbour's."
He encouraged farmers to report missing animals or other suspicious behaviour, as thieves also tended to target quad bikes and fuel when hitting farms.
New Zealand's national farming body has lashed out at the underground meat trade for fuelling stock thefts and selling unsafe meat.
Federated Farmers president Bruce Wills said those participating in the "pub trade" were to blame for rustling.
"The most distressing thing we have is the theft of our animals. It is a constant issue and a constant concern for rural New Zealand."
Mr Wills, a Hawke's Bay farmer, said rustling was sometimes carried out by gangs, often armed with guns and knives. It usually happened in waves and was punishing for any farmer.
"A good ewe is $100-plus and a good cattle beast is easily $1000-plus. So it's a significant financial loss to farmers, but it's also the emotional cost."
He warned those buying into the "hot meat" market, which was popular among pub owners for its cheap prices, were taking a serious health risk.
"We are required to keep detailed record about any animal remedies and drenching we give.
"There are strict protocols about when we sell animals [and] there are things we call withholding periods, which prevent us from selling animals within a certain period to being drenched and receiving remedies."
Meat from rustled animals could contain drenching chemicals, posing a serious risk to human health, he said.
In an effort to combat rustling gangs, a formal agreement between the Federated Farmers and police was signed last year.
"We now have a memorandum of understanding with the New Zealand police," Mr Wills said.
"Farmers need to be vigilant and report any stock which has gone missing or been taken ... and under no circumstances, should farmers take the law into their own hands as they [rustling gangs] often come armed with guns, knives and dogs."