Three prisoners are being crammed into one-man cells, self-harm has skyrocketed and inmates wait more than a month to see a doctor, according to a snapshot of overcrowding in NSW jails that warned of potential prison riots.

The alarming report by the independent NSW Inspector of Custodial Services, John Paget, said prisoners are kept in cramped conditions for long periods where they "shower, eat and defecate" which "inevitably raises tensions in an already volatile population".

It found prison infrastructure is inadequate and some inmates go without proper health care. The state opposition described the miserable conditions as "inhumane".

Higher arrest rates and tougher penalties have driven up the prison population over the past few years, and numbers are expected to increase further this year due to tough new bail laws.

There are about 11,500 inmates in the state's prisons. In February, Parklea Correctional Centre held 51 per cent more inmates than it was designed for and a section of Long Bay jail was about 70 per cent over capacity, the report said.