People buying cheap lamb this Christmas could end up with a poisonous roast.

Police in Horowhenua are concerned a recent spate of sheep-rustling is to supply the black market with lamb ahead of Christmas.

Federated Farmers Meat & Fibre Industry Group chairperson Rick Powdrell says stolen animals might've just been vaccinated, or drenched.

He says stolen lamb won't go through butcher's shops.

Mr Powdrell says they'll be sold from the back of the car, or the back of sheds, so it will be clear it's stolen meat.