A Palmerston mum who was approached by two armed male teens while putting her young child inside her car near Goyder Square had her purse allegedly stolen and was threatened with a knife for her car, NT Police said.

The incident occurred at an unspecified time Tuesday morning.

Police said after the youths stole the handbag, they produced her car keys and demanded she remove the child before pulling out the “edged weapon”.

That was when a passerby jumped into action and protected the woman and her child.

“A nearby witness came to the woman’s aid and restrained one of the youths until police arrived where he was subsequently arrested,” police said in a statement.

“The second offender fled the scene.”

Police said a 13-year-old male was in custody and expected to be charged last evening.

Any witnesses to the incident are being asked to contact police on 131 444, quoting reference number 10441867.