A pregnant woman has spoken of aiding a man as he lay badly injured and bleeding from a gunshot wound on a Christchurch bridge last night.

Police say a 29-year-old man was shot once in the shoulder about 8.20pm, near the intersection of Radley St and Ferry Rd, where a bridge crosses the Heathcote River.

It's understood the man was standing on the bridge when he was shot.

Responding to a post about the incident on Facebook, Jade Taiapo described how she'd "saved a man's life tonight".

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Taiapo said the man's sister had yelled and flagged her down as she was driving past.

She said she stayed with the man, holding pressure on his wound and counting with him, and didn't want to leave until the ambulance arrived.

A woman, who said she was the man's cousin, thanked Taiapo for her efforts.

Taiapo replied: "[You're] so welcome wasn't just me a man was doing all the holding on his neck cause I'm so big and pregnant this was horrible …"

A police spokesperson confirmed two members of the public who were in the area stopped to assist and provided medical attention until the ambulance arrived.

Armed police cordoned off the bridge last night until about 10pm, and took samples from bloody clothing.

Detective Sergeant Jo Carolan said the shooter was travelling in a vehicle and left immediately afterward.

The victim remained in hospital today, in a serious but stable condition.

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Police were following strong lines of inquiry, Carolan said.

"Police believe those involved in this incident are known to each other and there is no risk to the general public," she said.

"We would like to hear from anyone who was in the Radley St/Ferry Rd area around the time of the shooting, who may have information which could assist our inquiries."