Four male teenage detainees have threatened security officers with a knife before fleeing in a prison van on the NSW north coast.

At 4.30pm on Sunday police were called to North Street in Grafton where the teens allegedly threatened two security officers with a knife.

The security officers and another detainee got out of the van.

Police say four detainees, three aged 18 and one 15-year-old, then allegedly took off with the juvenile justice van.

The vehicle was found on Redgum Road shortly after.

However the detainees are still on the run.

The 18-year-olds are described with black hair, medium builds and between 150 and 170cm tall.

The fourth teen is described as being of Caucasian appearance, approximately 150cm to 155cm tall, with a thin build and brown hair.

Police have urged people not to approach the young men but to call 000 immediately.

AAP