A recently-graduated police officer has been praised for his bravery after rescuing a woman who jumped 15m from a bridge into the Waikato River.

The incident at the Cobham Drive bridge in Hamilton began about 11.30pm on Friday night when a woman was seen standing on the bridge's railings. While talking to police and ambulance staff, the woman jumped 15 metres into the water below.

The constable swam out to reach the woman and pulled her back to the river bank, police said.

Both were rescued by the police boat. The woman received CPR while she was transported to the boat ramp, where ambulance staff took over.

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She was taken to Waikato Hospital in a serious condition.

The constable was also taken to hospital and was released shortly after.

A police spokesman said the constable "put his own life at risk to save another person".

"Without the constable's act of bravery the woman's family and friends could be grieving the loss of a loved one."

Hospital spokeswoman Mary Anne Gill said the woman was still undergoing assessment.