There is concern after reported incidents of sheep rustlers using dogs and traps to round up, and capture sheep after dark at Auckland's Cornwall Park.
Police said members of the public caught three men and three large dogs capturing newborn lambs last Wednesday.
Cornwall Park staff were concerned for the welfare of the farm's stock, particularly in the lambing season and were working long hours to ensure the flock's safety, police said.
Police and park staff asked nearby residents and visitors to the park to call police on 111 immediately if they noticed any suspicious behaviour.
They would also like to hear from anyone who has witnessed these types of incidents recently, or who can identify the people and dogs involved.Anonymous information can be provided to Crimestoppers on 0800 555 111.