Auckland's rescue helicopters can now administer blood transfusions to critically-injured people at the scene of accidents or en route to hospital.
The new service is being given a three-month trial.
A medical doctor, Christopher Denny, said the first hour after a traumatic injury or medical emergency is critical.
He said research showed critically injured patients have a higher chance of survival if given a pre-hospital transfusion.
Mr Denny said blood would be transported in a mini cooler box that for safety and wastage reasons will be exchanged at Auckland Hospital's blood bank every two to three days.
The service - which is jointly led by New Zealand Blood Service, St John, the Auckland District Health Board and the Auckland Rescue Helicopter Trust - will be reviewed in March.