Canterbury Civil Defence is warning people to stay away from the Conway River after Towy River, which flows into Conway, was blocked by a slip.
The North Canterbury river is south west of Kaikoura.
A landslide had formed a dam in the Towy River and the waterway was in "imminent danger" of overflowing, Civil Defence said.
The agency feared the blockage would also cause the water level in the Conway River to rapidly rise and wash debris downstream.
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Civil Defence had "serious concerns" for the safety of anyone in or near the river, it said.