Civil Defence says it's concerned about settlements on the shores of the Hapuku River near Kaikoura and the Waima River in Marlborough.
Monday's massive earthquake has dammed a river in the hills near Hapuku, and a number of locals have been evacuated because of fears the dam will burst and flood their homes.
Pictures taken by a local resident show the moment the hundreds of tonnes of dirt slid down the Kaikoura range, creating a 150m-high dam.
While the water has not yet reached that height, a lake has built up behind the dam and, should it burst, it would send a wall of water downstream to Hapuku.
The dam on the Hapuku River (Canterbury Civil Defence / Supplied)
Environment Canterbury Regional Council senior river engineer Shaun McCracken says there are three scenarios that could happen as the river swells at the base of the dam.
"One is that it just builds up and overtops, but the whole thing remains stable, and we'll just be left with a new lake in the top of the catchment," he says.
"The second scenario is the worst one, where it's a rapid failure of the dam and all the lake water is discharged, which would be full of debris. If that happens, all the water will be contained within the river terraces, but the bridges down by the coast would be put at risk."
The moment the landslide that caused a build-up in the Hapuku River struck (Neil Protheroe / Supplied)
Mr McCracken said the third scenario is a combination of the first two, whereby the damage wouldn't be as bad, but there would be a changed landscape at the top of the catchment.
Authorities feared it could wipe out the only bridge north of Kaikoura on State Highway 1, but Mr McCracken says "even in a worst-case scenario", the town wouldn't be affected as the typography of the land wouldn't allow the water to flow that way.
Civil Defence is strongly urging people to stay away from the riverbed, and police have been warning campers and residents in the Hapuku area of the danger.
Mr McCracken says the potential danger could prove to be something they need to monitor over a number of weeks.
The moment the landslide that caused a build-up in the Hapuku River struck (Neil Protheroe / Supplied)
Meanwhile Civil Defence has also sent geotechnical engineers to look into landslide dams at the Waima River, in aftershock-ravaged Marlborough.
Those living in homes in the valley downstream of the dams have been told to evacuate by mid-morning on Friday.
Mr McCracken says ECan is also aware of another landslide dam that has emerged in the Conway River, between Canterbury and Marlborough, but it is of significantly less concern than those in the Hapuku River and Waima River.
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