A woman who witnessed a teenager try to jump off a bridge twice in one day is calling for the hospital which released her to investigate the incident.

At around 11.30am on Sunday, Christchurch resident Hannah Carter and her partner saw a woman standing on the Wrights Rd overpass in Addington.

Her partner Elliot Sizer ran up the bank while she called emergency services.

Cater said her partner spoke with the teen then grabbed her and pulled her back as she got closer to the edge.

"We were on our way to do a bit of shopping and noticed a female up there. Something just didn't seem right so we pulled over and sure enough, it became obvious that she was contemplating suicide."

"My partner ran up there, tried to talk to her then grabbed her by the backpack and she removed her backpack, proceeded to try to jump and he grabbed her by the arms and pulled her over the barrier."

"Then we pulled her down, we talked to her, we actually had a puppy in the car which was quite good and calmed her down a bit."

Speaking with Mike Hosking, Carter said the distressed woman told them she was 19-years-old and had deep self-inflicted cuts on her arms.

"She displayed quite a few open wounds on her arms and she also advised that one of her wounds she had just done, I won't go into details but it was pretty nasty."

"She was obviously very distressed."

Carter was relieved when police showed up, and they told her they'd be taking the woman to Hillmorton Hospital - a mental health facility - for urgent care.

"We just thought thank goodness, she's obviously going to receive the urgent care she desperately needs."

But just two hours later as they were driving home, Carter saw police and other members of the public trying to talk the same woman down from the same bridge – in an even worse state with "blood on her face".

"I drove down that same road and noticed that there was a handful of cars and cops behind us, and I looked behind me and there she was again...up on the bridge."

She said she's shocked the woman, who clearly needed help, was released from the hospital so soon.

"It's not just about putting her life at risk again, it's also members of the public that could have been driving under that overpass...putting their lives at risk."

"She's a girl who is screaming out for help and she's been taken to the one place where she is supposed to receive that, and she's just been turned away."

"It just seems absolutely crazy that someone in that state of mind was released within a matter of hours and allowed to get up there again."

Carter said it's ridiculous the hospital released the teenage within a couple of hours.

"I did contact Hillmorton briefly, just to let them know what happened, let them know what we witnessed and unfortunately their response was quite weak. They didn't ask for any details, it was just I'm sorry this happened to you, so yeah it was quite disappointing."

She said she hopes Hillmorton Hospital investigate what happened.

Carter said the police officer she spoke to seemed concerned about the incident as well.

In a statement to NewstalkZB, a spokesperson for the CDHB says whenever there is a significant event or near-miss, staff review what happened, the circumstances at the time of the event and what, if anything, could be done to prevent a similar event happening in the future.

It says they're unable to comment on individual cases due to privacy reasons.

Where to get help:

If you are worried about your or someone else's mental health, the best place to get help is your GP or local mental health provider. However, if you or someone else is in danger or endangering others, call police immediately on 111.

Need to talk? Free call or text 1737 any time for support from a trained counsellor.

Or if you need to talk to someone else:

Lifeline – 0800 543 354
Suicide Crisis Helpline – 0508 828 865 (0508 TAUTOKO)
Youthline – 0800 376 633 or free text 234
Kidsline – 0800 54 37 54 (for under 18s)
What's Up – 0800 942 8787 (for 5–18 year olds 1pm–10pm weekdays and 3pm–10pm weekends)
Depression Helpline – 0800 111 757 or free text 4202
Samaritans – 0800 726 666
OUTLine NZ – 0800 688 5463
Healthline – 0800 611 116