The SPCA has concerns about a policy of Whangarei District Council of shooting wandering stock and believes it may be against the law.
The council now destroys animals which repeatedly escape from their paddocks, but says it has given the owners plenty of warnings.
The SPCA says the law that applies is the Impounding Act, and can't see how shooting the animals can be legal.
It says the Act allows councils to destroy sick or worthless stock, but not without notice to the owner.
The council says it can shoot wandering stock, under Section 42 of the Impounding Act, when it's not practical to recapture them, and those that were shot, posed a fatal risk to motorists.
Environmental services manager Paul Dell says that by adopting a hardline stance, council was hoping to make owners take responsibility for their animals and the policy seems to be working.
Since the shooting policy began, Mr Dell says calls about wandering stock have dropped from 40 a month to five or six.
Police in Northland are also appealing to landowners to keep their stock off the roads, after a spate of collisions with cattle.
The police say they have had 60 complaints this month about wandering stock, and three crashes involving animals.
Senior Sergeant Geoff Ryan, of the Kaitaia Police, says in the most recent crash a woman was lucky not to be killed when she hit a cow on the Awanui straight.
He says in crashes where the animal is thrown up onto the car the driver's chances of survival are low.
The Far North District Council says the increase in wandering stock complaints may be linked to the drought, with farmers who are short of feed grazing their stock on road verges.