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Police were called about 6.45pm when the man's friends were unable to locate him after he had gone for a swim near the Waimakariri Bridge.
A spokesperson said the Police National Dive Squad would arrive this morning to "assist in the search for the swimmer missing in the Waimakariri River".
"Their search efforts will focus on the area between the two bridges, the State Highway 1 bridge and the Old Waimakariri bridge."
Police said a search of the river with assistance from the Kaiapoi Coastguard did not find the man.
Anyone with information about the incident is asked to call 105 and quote file number 210130/5446.