The NSW prisoner population is at a historic high of 11,100, with inmates having to be detained overnight at police stations and courthouses because there are not enough jail beds to meet the current demand.

And the number of inmates is only expected to increase, with Corrective Services reporting the unprecedented surge in numbers began in December, a month before the NSW government's tougher bail laws came into effect.

A department spokesman said the prison population had been rising for two years and was now "the highest it has ever been" and close to capacity.

But the prison officer's union, the Public Service Association, said NSW could only accommodate 10,800 prisoners. PSA general secretary Anne Gardiner said NSW's prison system was in crisis, and called for Grafton jail, closed in July 2012, to be immediately reopened.

The NSW Police Association said it was concerned that police officers now had the additional responsibility of caring for inmates.