At least six inmates have surrendered during a riot where makeshift weapons have been used to “target sex offenders”, leaving several injured at a juvenile justice centre on the NSW 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 on Monday.

Several inmates remained on the roof at the complex on Monday afternoon – some 17 hours after the riot started – with some playing with sporting equipment.

Four inmates surrendered on Monday morning and two more climbed off the roof after 1pm.

It’s unclear how many inmates remain on the roof, with police saying between 10 and 20 were “not where they should be” earlier on Monday morning.

The union that represents prison guards said the offenders stole keys from an officer and took garden tools, including a hedge trimmer and shovels, from a maintenance shed at NSW’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 said.

“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 was not the first riot at the facility.

Detainees on the roof of the centre on Monday. Photo: AAP

Police are continuing to negotiate, working alongside the riot squad, dog unit, helicopter and rescue unit.

They have established a secure perimeter inside the centre, which has a capacity of 120 young people, and quarantined the armed detainees in a building within the complex.

“It seems a very sudden and unprovoked incident,” Superintendent Tony 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.

-AAP