A 19-year-old man in Katherine was killed in a stabbing overnight. (Supplied: NT Police Force)

Northern Territory Police say they expect to charge a 19-year-old man with the stabbing murder of another 19-year-old man in Katherine last night.

Investigators say a third man, 18, was also allegedly stabbed by the same man and airlifted to Royal Darwin Hospital with significant injuries, where he remains in a critical condition.

This afternoon, Assistant Commissioner Peter Malley said police had responded to a call for help on Holtze Crescent in Katherine East about 6:45pm and arrived to find the fatally wounded man sitting in a car.

“The officers present could see that he was bleeding profusely and was in a pretty bad way,” he said.

Assistant Commissioner Malley said the officers called an ambulance but decided to take him to the Katherine Field Hospital, set up due to recent flooding, in the back of their caged wagon.

“[The ambulance] was about 10 minutes away, so they made the decision to transport him, it was going to be quicker as they were only about 3 minutes away from the hospital,” he said.

“He was conscious and breathing at that stage when he was in the back of the van with the two police and they applied pressure to his wounds in an attempt to stem the bleeding.”

The man was unable to be revived and the incident is being treated as a death in custody due to his prior interaction with police.

Assistant Commissioner Malley said the 19-year-old’s death followed “a fist fight between two males” before others became involved, including the alleged offender, who police allege also stabbed the 18-year-old.

He said the alleged offender then drove the injured 18-year-old man to the field hospital where was arrested after allegedly discarding “a significant sized knife” on the way.

A post-mortem is due to be conducted later in the week.

Assistant Commissioner Malley said he expected the alleged offender would be charged with the other 19-year-old’s murder.

His death is now the NT’s third death in custody in less than a week.