A person who caught two men breaking into their vehicle early this morning is in hospital, following an altercation which also left one of the alleged thieves with a life-threatening head injury.

Police were called just after 4am to a report of a vehicle break-in at a property on Te Atatu Rd in West Auckland, close to the northwestern motorway.

A spokeswoman said a person had reportedly found two men breaking into their vehicle.

"During the ensuing altercation one of the alleged offenders has left the scene in a vehicle, and the second sustained a head injury and was transported to hospital in a critical condition," she said.

A scene guard is in place. No charges have been laid and no one has been arrested, police said.

A St John spokeswoman said two people had been hospitalised - one was taken to Auckland Hospital in a critical condition and another person had been taken to Waitakere Hospital in a moderate condition.

Emergency services could not tell the Herald whether the second person was injured or had suffered a medical event.

A person at the scene said two police cars were still in attendance around 6am. An orange sports shoe and a bloodied towel or sheet was visible in the driveway, he said.

Police are still making inquiries and looking for the other person involved.