Labour leader Phil Goff says any change to the management structure of the Waikato River would necessitate a renegotiation of the Treaty agreement governing the waterway.
Waatea News reports iwi and regulatory bodies last year agreed to setting up six boards to co-ordinate a $210 million clean up of the river.
But Mr Goff says a panel review conducted for Treaty Negotiations Minister Chris Finlayson recommends having a single board.
A spokesperson for the minister says there would be no change unless iwi agree - and any amendments to the deed of settlement would go before a select committee.