Reforesting eroding hills will keep soil on the land and out of rivers and help fight climate change, the Parliamentary Commissioner for the Environment says.
Dr Jan Wright has released a mixed report card on the state of New Zealand's environment.
She found that climate change was by far the most serious environmental issue.
"Already temperatures are increasing, the ocean is acidifying, and the sea is rising," she said.
The Commissioner also found that water quality, pests, and erosion were high priorities for action.
"While water quality is good in forested areas, it is poor in many lowland places. Our native plants and animals are losing the war against pests. And we still have huge areas of eroding hill country."
She found New Zealand's oceans were under pressure from run-off and fishing, although the greatest pressure in the long-term was acidification and warming from climate change.
In contrast, the outlook for air quality is good - it looks set to continue improving.
She recommended the Secretary for the Environment make a formal response, saying how the Environment Ministry will deal with the issues raised.
Under the Environmental Reporting Act, the Commissioner provides commentary on reports prepared by the Ministry for the Environment and Statistics New Zealand. Dr Wright commended the Ministry and Statistics New Zealand on what they have achieved in a limited amount of time, but identified eight areas for improvement.