The Rivers Agency has said it got no advance notice of plans to drain a dam in County Antrim despite asking for it.
A digger was used to pull out part of the retaining wall at the privately-owned facility near Mallusk.
But the company responsible, Hyde Park Dam Ltd, said it did have permission to carry out the work.
The agency wrote to all dam and reservoir owners on 28 March asking them to put in place plans for "safety-related works".
It said this was to prevent flooding caused by any potential breach.
That was to comply with new regulations on reservoirs.
The agency also asked for advance notice of any proposed work.
In a statement, it said it had subsequently become aware that work had been done at Hyde Park dam.
"We had received no prior notification of these works," the statement said.
"We are currently seeking clarification from the reservoir manager."
A large volume of water flowed out of the dam and into the Ballymartin River, a tributary of the Six Mile Water.
Locals complained that draining the dam had an impact on nesting birds.
The police confirmed it was investigating potential breaches of wildlife legislation that protects wild birds.
The company said it had engaged an ecologist to inspect the site and they had found no evidence of "undue impact or disruption to wildlife on the site".
It also said it did have Rivers Agency approval to carry out the work dating back to November 2016.
Hyde Park Dam Ltd said it received notification from Rivers Agency on March 28 2017 "advising us that we should start work to control the possibility of flooding".
It said this work had been done "swiftly and responsibly" to ensure public safety.