The Animal Health Board says someone is deliberately destroying signs warning people about the presence of 1080 poison in Westland.
The board says contractors targetting possums in the area had put up warning signs in the Mikonui-Totara area, just south of Hokitika.
They have been destroyed over the last week.
Signs further south at Waitaha, which warned that cyanide had been laid, have also destroyed.
The vandalism follows anti-1080 protests on the West Coast last month.
The board's area programme manager, Chris Pullen, says the vandalism is putting the public at risk, and the matter has been referred to the police.