Rimutaka Prison - the country's largest prison, is adding more beds to keep up with the growing number of inmates.

The country's largest prison is expanding its capacity with the installation of "modular units" in an effort to keep up with the growing number of inmates.

Corrections began installing two Chinese-made 122 bed accommodation units for low-security prisoners at Rimutaka Prison in Upper Hutt in December. The new units will increase the maximum capacity of the prison from 1118 to 1362.

Eight of the units are being installed at five Corrections sites across the country with one unit each for Christchurch Woman's and Tongariro prisons, and two each for Rolleston, Christchurch Men's and Rimutaka prisons.

Phil Reid ontainer cells were added to Rimutaka Prison in 2010.

Each of the units consists of 60 double-bunked cells, two single-bunked cells and other spaces for prisoners and staff such as laundry rooms and offices.

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Rimutaka Prison director Vivian Whelan said the increased capacity would ensure they would be able to react to future fluctuations in the prison population.

The number of inmates in the country's prisons had increased 20 per cent over the last five years, peaking at 10,820 in March last year. There were currently below 10,000 inmates held in New Zealand's prisons.

It was reported last year quality issues had been discovered while the units were being made.

Corrections chief custodial officer Neil Beales said off-shore manufacture of modular prisoner accommodation was a first for New Zealand and "minor issues" were identified during manufacturing but had been addressed by the supplier.

"These were typical of the types of issues that occur in large-scale construction work of this nature and included minor design changes, standardised work quality across the units and other matters relating to finishing. The units that have arrived in New Zealand to date have been inspected, with some remediation to superficial plaster work required due to movement during transit."

The units were made of "high quality components" which met New Zealand building code standards. A quality assurance company had also been contracted to test materials and monitor the manufacture of the cells, Beales said.