About 20 people have been evacuated from southern Marlborough because of fears that several slip dams created by Monday's 7.8 earthquake will breach.
Marlborough civil defence controller Richard McNamara with a map of the Ure River area. Photo: RNZ / Kate Newton
Three dams have formed along the Ure River, about an hour south of Blenheim, from slips and debris.
The evacuated houses are clustered about 9km upstream from the river mouth and State Highway 1, which remains closed.
Marlborough civil defence controllor Richard McNamara said geotechnical staff had inspected the dams and water was building up behind them.
"One of the dams is upwards of 35 metres high. If they let go, then there's going to be a wall of water come down the river."
About 20mm of rain forecast for this afternoon would put extra pressure on the dams.
"So last night we rang and physically knocked on doors and asked people to move," Mr McNamara said.
"We're just confirming that this morning by having an aircraft go through."
The dams would break sooner or later, he said.
"The worst-case scenario is that the top one goes and it just cascades as it goes down. At that point it ceases to be just a heavy flow of water - it becomes a debris flow."
A breach could also take out the Ure River bridge on State Highway 1, blocking the way of emergency and other lifeline vehicles that were using the closed highway.
"If the debris flow comes down river ... it could further weaken the bridge, if not take it out, so that's a real issue for us."
Civil defence was also trying to establish whether or not any children living south of the river had travelled north to the settlement of Ward, where the local school had re-opened.
"If kids are at school ... it's likely we'll hold them at Ward until such time as we have cleared the danger."