SYDNEY • At least seven people were injured after detainees armed themselves with makeshift weapons and assaulted other prisoners during a siege in an Australian juvenile justice facility at the New South Wales (NSW) Central Coast late on Sunday.

The ongoing incident at the Frank Baxter Juvenile Justice Centre began on Sunday evening, involving up to 20 male inmates aged 16 to 20 who have all been detained, NSW Police said.

Around four of the detainees involved in the riot reportedly surrendered yesterday morning after spending the night on the roof of the facility in Kariong in near-freezing conditions, Xinhua news agency reported. The siege - said to be part of a plot to murder or badly injure sex offenders - ended after about 21 hours, according to Australia's 7news.

Six inmates - two stabbed and four badly beaten - are in hospital. A seventh inmate was treated at the scene for a broken nose.

Staff and detainees were attacked after the offenders stole keys from an officer and armed themselves with gardening tools, including a hedge trimmer and shovels, from a maintenance shed at the state's largest juvenile justice centre, 7news reported.

Families and Communities Services Minister Gareth Ward yesterday called the incident "unacceptable".

"We are treating it very seriously," he said.

"Our priority is the safety and well-being of staff and detainees involved... Actions that put the safety and security of staff and detainees at risk will not be tolerated."