The prison population reached a record high of 10,917 in March.

The prison population reached a record high of 10,917 in March. Photo: Andrew Meares

Two large, Parklea-sized prisons would need to be built each year in NSW to house an exploding prison population that advocates warn could lead to riots and public expenditure blowouts.

An unprecedented rise in the prison population in the past year will only worsen if mandatory sentences for drunken assaults are introduced, former Director of Public Prosecutions Nicholas Cowdery said.

The prison population rose by 13 per cent in a year to reach a record high of 10,917 in March, a NSW Bureau of Crime Statistics and Research report released on Thursday found.

Even without mandatory sentencing, BOCSAR predicts it will soar a further 17 per cent this year, creating 2000 new prisoners who would fill Parklea Correctional Centre twice.

BOCSAR director Don Weatherburn said the rapid rise was ''a matter of significant concern'' not only because each prisoner costs $119 a day but because the prison population was rising faster than the government could build capacity, creating the risk of prison unrest.

More than 100 prisoners at Parklea signed a petition in March after management started placing three men in two-man cells, leading to increased tension and violence.

Attorney-General Brad Hazzard spruiked the rise in prisoners as a win for ''community safety'' on Thursday despite his predecessor Greg Smith vowing in 2011 to reform the prison system and reduce Australia's largest jail population.

When prison numbers hit 10,000 under the Labor government, Mr Smith called it a ''disgrace''.

Mr Hazzard conceded that finding room to house prisoners was ''a constant challenge'' and he was unable to mention specific plans to ease the problem.

The BOCSAR report found that crime was decreasing in NSW but more people were being arrested, prisoners were remaining in jail for longer and more offenders were receiving prison sentences for offences such as driving while disqualified.

''That's going up because the number of people who don't pay their fines has gone up, so what looks like a driving while disqualified conviction is really just a fine default,'' Dr Weatherburn said. ''Is this really the way we want to deal with fine defaults?"

He said prison was not a cost-effective way of reducing crime and its effectiveness was further reduced when prisons were crowded. NSW also has one of the worst recidivism rates in Australia.

In a speech to the Sydney Law School on Thursday night, Mr Cowdery, a University of Sydney adjunct professor of law, said more prisons will have to be built and public expense will increase ''enormously'' if mandatory sentences for alcohol-related assaults are introduced.

The Parliament will debate the proposed laws again this month.

Brett Collins, co-ordinator of prisoner advocacy group Justice Action, said Parklea was a ''tinderbox waiting to explode''.

''Putting more people in prison will only have the opposite effect of what is intended,'' he said. ''It is an incubator for crime.''

A Corrective Services NSW spokesman said they have the capability to deal with increasing prison numbers by adjusting the use of space in existing facilities and reviewing bed occupancy.