Otago Regional Council is concerned about a growing trend in the region for farm dams being built illegally.
Under the Building Act, any dam that deeper than three metres and holding more than 20,000 cubic metres of water requires resource consent.
The council's environmental engineering director Gavin Palmer says the council has found a number of farm dams that require this consent but don't have it.
He says it's a significant issue which could be part of a nationwide problem.
Dr Palmer says any farmer considering building a dam should seek professional advice, and check whether consent is required under the Resource Management Act or the Building Act.
Canterbury Regional Council says this month's earthquake has increased attention on farm dams in its area.
The director of regulation, Kim Drummond, says any property containing a large volume of water poses a potential risk to neighbours and nearby land and the quake has put stress on some of these structures.
He says the council is working with two landowners who have built large dams illegally without resource consent.