NSW police investigate juvenile detention centre stand-off

Police will investigate after two inmates spent more than nine hours on the roof of a western Sydney youth detention centre.

The young males climbed onto the roof of a unit at the Cobham Juvenile Justice Centre in St Marys just after 10am on Monday and came down voluntarily about 7.30pm.

While staff tried to persuade them off the roof, the males broke off pipes and parts of a metal air conditioner, which they brandished and threw around.

The lengthy stand-off appears to have begun with an "impulsive act", according to a Juvenile Justice spokeswoman.

The males were retaken into custody and the matter was referred to police, who are now investigating.