An MP is calling for the minister to step in and question why Eastland Group has not been prosecuted after a sediment spill polluted the Waihi and Waiau rivers.

Green Party water spokeswoman Catherine Delahunty said there were serious questions to ask of the regional council about why it had not prosecuted the Eastland Group before now and why its investigation is taking so long.

She wants Environment Minister Nick Smith to ask those questions.

"It's the minister's job to ensure regional councils do their job and protect the environment."

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Waihi Dam has been discharging silt into Waiau River since before Christmas. The discharge began after a sluice gate was damaged in a storm late last year. The council issued an abatement notice on Christmas Eve and January 6, requiring the gate to be closed, but the damage to the gate is preventing it being fully closed.

The council was told last week that Eastland Group had now regained control of the dam.

Council resource management group manager Iain Maxwell said no decision had yet been made as to whether there would be a prosecution in relation to the silt discharge at the Waihi Dam.

Eastland Group had completed the repairs to the gates and all three sluice gates were now closed. The reservoir was filling and, once full, power generation would begin again. The abatement notice was lifted.

Mr Maxwell said there were statutory timeframes within the Resource Management Act stipulating that any prosecution against any party must be initiated [filed with the Environment Court] within six months of the regional council becoming aware of the incident.

"The council has always operated within those statutory timeframes and will do so in this case also."

Decisions to prosecute or to take any other enforcement action are made without political influence or pressure, in line with the Solicitor-General's Guidelines for Prosecutions.

"Calls from politicians to prosecute are neither helpful, nor appropriate," Mr Maxwell said.

The primary focus for the past four months had been to see Eastland Group close the gates at the dam and stop the flow of silt into Waihi Stream.

At the same time, a thorough investigation had been under way and was continuing into how the situation occurred and what effect it was having on the community and the environment.

The investigation was being undertaken by the council's Resource Use team, headed by a manager with more than 40 years' experience in investigations and enforcement.

Mr Maxwell said no stone was being left unturned in relation to understanding the circumstances surrounding the incident.

Once a decision was made on whether any party should be prosecuted, that party would be advised and that decision would be made public, which was expected toward the end of April/beginning of May.

Mr Maxwell said the decision would not be rushed at the expense of the right decision being made.