Flynn Parker was at the scene said there were around three to four police cars and two ambulances involved in the rescue at the popular rope swing site.
"A helicopter came in very swiftly and was searching for two hours back and forth in the trees. Around four boats, including the harbour master and kayaks, etc, all were looking in the water."
He said volunteers joined the search and police watched from the side.
"We didn't know anything was happening until the helicopter came swiftly in and hovered very low to the ground, that was around 7.15-7.30pm."
He said kayaks, boats and a bunch of people on little orange boats had been searching for the swimmer near the rope swing.
"Not looking very hopeful sadly."
A spokesperson for St Johns ambulance said earlier a rescue helicopter, police and St John were searching for the swimmer.
Photos from the scene show a rescue helicopter hovering over an area of the river while several people watch from the river's edge.
Another shows a rope swing hanging from a tree near the water.
Two rescue vessels from Coastguard North Canterbury were assisting police with the search.
The search comes after a child died in a suspected drowning earlier today at a Christchurch pool.
A distraught woman was heard yelling "that's my baby" as a child was pulled unresponsive from the water.
A police spokesperson said emergency services were called to Waltham Swimming Pool in Sydenham at 2.45pm after reports a child had been found unresponsive.
"CPR was carried out, however tragically the child was unable to be revived. Inquiries into the circumstances are under way."
A traumatised woman who was at the pool with her 8-month old baby told Stuff she was in the toddler pool when she heard the lifeguard's whistle.
She initially thought someone was breaking the rules.
"But then a lady on the grassy hill ahead of me screamed 'that's my baby' and she and a group of people ran from the grass to the pool, where a child was being pulled out."
The woman said up to 30 people crowded around while the lifeguards tried desperately to revive the baby, she said.
Nigel Cox, Christchurch City Council head of recreation, sports and events, said the pool will be closed today and tomorrow.