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Inmates used makeshift weapons to "target sex offenders" in an ongoing riot that has left at least six people injured at a juvenile justice centre on New South Wales' Central Coast.
The riot broke out at Frank Baxter Juvenile Justice Centre at Kariong about 8.30pm on Sunday, with reports several inmates assaulted each other, NSW Police said in a statement on Monday.
The union which represents prison guards said the offenders stole keys from an officer to obtain garden tools including a hedge trimmer and shovels from a maintenance shed at the state's largest juvenile justice centre.
Public Service Association general secretary Stewart Little said the group targeted known sex offenders within the complex and described the riot as "nothing short of horrific".
"We have a juvenile justice system that has descended into chaos," he told reporters in Sydney on Monday.
"Up to 30 detainees have obtained control of that centre, they have targeted known sex offenders with an intent to murder or do very serious harm to those offenders."
He called for a judicial inquiry into the juvenile system noting it's not the first riot at the Kariong facility.
Between 10 and 20 inmates are still "not where they should be" more than 12 hours after the riot started, Superintendent Tony Joice said.
Several inmates remain on the roof at the complex with some playing with sporting equipment.
Four inmates surrendered on Monday morning as police continue their negotiations alongside the riot squad, dog unit, PolAir helicopter and rescue unit.
Police have established a secure perimeter inside the juvenile justice centre, which has a capacity of 120 young people, and restricted the armed detainees to a building within the complex
"It seems a very sudden and unprovoked incident," Supt Joice said.
"Identifying the means and reasons why the assaults have taken place is part of the investigation."
An 18-year-old inmate has stab wounds and facial injuries and is in Royal North Shore Hospital in a serious condition.
Four others, aged between 17 and 20, also suffered stab wounds and have been taken to Gosford Hospital for treatment.
A 17-year-old youth has been taken to the same hospital after dislocating his ankle when he jumped off a roof and another inmate was treated at the scene for a broken nose.
All inmates have been accounted for, police said.