Farmers in Southland are being warned that their dairy effluent storage ponds may not meet the regional council's standards, and may need to be rebuilt.
The regional council, Environment Southland, will write to about 200 farmers in coming weeks to ask whether the design and construction of their ponds was supervised by someone appropriately qualified.
It says staff have identified problems with the ponds, including inappropriate material used for lining, inadequate soil preparation and unstable sides.
Environment Southland says a farmer was recently told he would have to pay between $20,000 and $30,000 to make his pond comply with council standards.
Chief executive Ciaran Keogh says rather than prescribing how the ponds should be built, the council asks farmers to build a water-tight pond with a sufficient capacity. But he says some do not meet the criteria.
Mr Keogh says the council advises farmers to get a competent engineer to advise them and confirm the ponds have been built to whatever specification the engineer requires.
He says the soil across Southland is variable, so it is impractical to apply a single answer to the construction of effluent ponds.
Mr Keogh says the council's approach seems to be working for 90% of farmers.
However, Federated Farmers' dairy chair in Southland, Rod Pemberton, says when the council originally told farmers they would need to build effluent storage ponds there were no guidelines for them to follow, so it is no wonder some got it wrong.
Mr Pemberton says Environment Southland needs to take some responsibility for substandard ponds. He says storage needs to be revisited because although it has benefits, there is a huge environmental risk surrounding the ponds.