A jail in the New South Wales central west is set to be expanded to deal with the scores of inmates in the state prison system, as part of a $3.6 billion investment, the Government says.

Key points: New facility with 400 beds to be finished in 12 months

New facility with 400 beds to be finished in 12 months Corrections jobs to be doubled to about 400

Corrections jobs to be doubled to about 400 Prison expansion part of $3.6b investment, minister says

The expansion will involve the construction of a new, separate high-security facility on Wellington Correctional Centre land.

Corrections Minister David Elliott said he expected works would start as soon as next week, with an extra 400 beds to be added.

An extra 220 jobs will also be created, doubling the current work force at the prison.

"It's a big investment, it's a big boost to Wellington," Mr Elliott said.

"It will see a doubling of jobs in the corrections portfolio in Wellington to about 400.

"And of course the multiplier effect for the Wellington economy will be quite significant as well."

Wellington will be the first prison in the state to be expanded using "rapid build", pre-fabricated materials which will see construction completed over the next 12 months.

"[We will use] rapid build [construction] because we do have a lot of strain on our prison population at the moment," Mr Elliott said.

"It will see a new style of dormitory, it will see new programs which have inmates outside of their cells for about 12 hours a day."

Expansion part of $3.6b investment: minister

The Corrections Minister said there was "no doubt" the Wellington expansion would be the first of a number of other similar "rapid build" constructions.

The project is expected to add an extra 400 beds to the prison. ( Supplied: NSW Government )

"This type of investment is not just going to be restricted to Wellington, there's also — as a reflection of the way that our law and order portfolio is travelling at the moment — we'll be having more than one of these types of facilities," he said.

In June, the State Government announced a plan to bring almost 3,000 new prison beds into the state's system over the next four years.

It includes new beds at the Grafton, Cessnock, South Coast, Berrima and Wollongong correctional centres.

At the time, Mr Elliott said the Government was also planning to bring an additional 4,200 beds into the system over the next five years - but he did not reveal where.

The prison population has reached a record high of more than 12,000 inmates, with Corrective Services placing extra mattresses in cells at some jails to deal with the crowding.

In May, the Government scrapped a plan to reopen Parramatta jail, announcing it would instead sell Long Bay jail and build a new prison at Wollondilly, on Sydney's south-western outskirts.