In the video, a large chunk of a cliff falls into the river, causing a large tsunami which heads down the body of water. A second video sent to Newshub shows the river raging with muddy water.

It comes after intense rain overnight and on Saturday morning leading to flooding near the Rangitata township.

A state of emergency has been declared in the Timaru district, with the local Emergency Operations Centre being activated due to concerns about flooding. Residents have been told to evacuate from streets near the river.

Due to the widespread flooding risk, State Highway 1 at the Rangitata bridge and the State Highway 72 bridge near Arundel have been closed.

There are no diversions in place, so residents are being told to stay away. The council said the "disruption is likely to continue for the weekend", while NZTA's Pete Connors said the State Highway 72 route may not reopen on Saturday.

Civil Defence Information Centres are set up in Geraldine and Temuka.