A large volume of water is moving down the Waimakariri River carrying logs and debris following the rainfall on the West Coast, police say.
Anyone swimming or four-wheel driving in the riverbed area is advised to leave.
This covers the area between the Waimakariri Gorge and the sea.
As of 6pm, water was moving bank-to-bank through the gorge and could pose a threat to anyone in the flow area.
There is currently no risk to anyone outside of the riverbed area.
Police are asking people to follow the advice of rangers and police.