UP to 300 prisoners have been placed in lockdown and tear gas used to quell a riot and fire at the South Coast Correctional Centre in Nowra.
The riot broke out at the maximum security wing just after 9.30am.
Fire crews believe tissue paper and mattresses were used to set ablaze at least five cells.
Tear gas was being used to subdue inmates and dogs were deployed to the prison following the fires.
Prisoners have been placed into lock-down.
Rural Fire Service brigades, Fire and Rescue NSW, Police Rescue and Ambulance NSW crews are at the scene.
A media exclusion zone has been set up at the jail perimeter.