Work has started to halt the discharge of sediment from Waihi Dam near Wairoa in Hawke's Bay.
The Eastland Group dam has been discharging silt into the Waiau River since November after a sluice gate was damaged.
The Hawke's Bay Regional Council issued abatement notices on Christmas Eve and on 6 January requiring the gate to be closed, but the damage to the gate had been preventing it.
Council group manager of resource management Iain Maxwell said it was expected to take another three weeks to stop the sediment discharge.
"Not only does the work involve clearing the sediment from under the sluice gate to enable it to close, but also the development of a second structure downstream of the dam to trap as much sediment as possible from spilling out before it gets into the Waiau River," he said.
"A similar structure put in place a fortnight ago has been capturing a lot of sediment and a second is needed while the work is completed."
A thorough investigation into the cause and effect on the community and the environment was under way, Mr Maxwell said.
"A decision on whether Eastland Group will be prosecuted will be made once all the facts are known and the investigation is completed."