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.
Powdrell said rustlers should be wary of eating the meat from any animals caught in this way.
"At this time of the year a fair number of livestock are receiving animal remedies, as such, they are under a withholding period and until it clears their system should not be eaten.
The Federation wants legislation tightened and penalties around rustling introduced.
"We have been in talks with politicians and there seems to be increasing support to stiffen penalties for rustling.
Farmers can provide information anonymously through the Crimestoppers line on 0800 555 111.