Federated Farmers say that rustlers have targeted sheep on Auckland's Cornwall Park.
"A pregnant ewe caught in a gin-trap on Cornwall Park shows the lengths rustlers will go to," says Rick Powdrell, Federated Farmers rural security spokesperson.
"As we're on the cusp of main lambing I was shocked that they had targeted pregnant ewes but they probably do not care."
Powdrell said the current law offers no deterrent.
"We've seen sob stories being rewarded with community service and a few dollars in reparations. This doesn't reflect the gravity of the crime or the likely suffering of an animal being slaughtered by a rank amateur.