Waikato Regional Council says future conversions of land from pine trees to pasture in the Upper Waikato River catchment could increase the risk of flooding of streams feeding the river.
A council draft report says the findings are based on projected conversions of just over 500 square kilometres.
So far about 200 square kilometres has been converted.
The council's catchment services group manager, Scott Fowlds, says the report raises a number of issues that must be considered in dealing with land-use change and potential increases in flooding.
He says larger culverts may be needed to deal with additional flooding, and there may be more erosion in streams which will have to be managed.
Mr Fowlds says some areas will be identified that should not be cleared. The issues raised in the report will need to be discussed with interested parties such as iwi, farmers and local councils, he says.