People are being urged to stay away from the Hapuku River as a 150m-high dam threatens to fail.
The Canterbury Civil Defence Group this morning advised the dam could "rapidly fail" because of recent heavy rain and rising pressure, spilling water and debris from a newly formed lake over or through the dam.
"We strongly advise people to stay away from the riverbed, especially as the situation could change quickly with further earthquakes and/or rainfall," the group warned.
Police and others have been out identifying any campers and residents in the area and warning them to stay clear of the river and moving anyone on that is in harm's way.
The Hapuku River begins in the Seaward Kaikoura Range and flows south-east to enter the South Pacific at Hapuka, between Clarence and Kaikoura.
Meanwhile, occupants of four houses on the Ure River were advised to evacuate their homes before dark last night because of the risk of a surge of water from several dams created by landslide debris upstream.
One of the dams is just 1.5km from one of the houses.
The Ure, also known as the Waima, is south of Ward.
Civil Defence controller Richard McNamara said forecast rain over the next two days necessitated the move.
"We were able to advise 13 people that, for their safety, they should immediately move from their homes, and they have done so."
The houses are about 9km upstream from the river mouth and State Highway 1, which remains closed.