There's been a spate of sheep rustling in Auckland's Cornwall Park over the past few weeks.
Police said there had been a series of incidents where people had used dogs and traps to round up and capture sheep after dark.
They said last Wednesday members of the public disturbed three men and three large dogs taking new born lambs.
Cornwall Park staff were concerned for the welfare of the farm's stock, particularly in the current lambing season.
Police are asking nearby residents and visitors to the park to be extra vigilant and to report any suspicious behaviour.