Police received a report just before 5.30pm on Sunday that a man had fallen from a raft into the river at an area known as Griffin Flat, approximately 25km east of Ahaura, at about noon.
A police spokesperson said the group of four were travelling on two rafts.
One of the man's companions raised the alarm by phone but the area is extremely remote and cellphone reception was an issue, they said.
A search operation commenced at about 7am on Monday morning involving police, LandSAR and other volunteers.
Approximately 17 people were assisting with the search.
Three field teams are searching the riverbanks, a jet boat is searching the river and a helicopter is conducting an aerial search.
The Police National Dive Squad will also be assisting with the search.
