Prisoners at the Otago Corrections Facility (OCF) are not sharing cells, but that could change if it gets closer to full capacity.
The Public Service Association is warning that "double-bunking" prisoners will "increase tensions and heighten the risk of violence among prisoners and against staff".
As of last Friday, the prison population at the Milburn facility was 295 - just 40 shy of its 335 limit, and the Department of Corrections has hinted the time is getting closer to when some prisoners there could be sharing a room.
OCF has 35 cells with two beds, and the Public Service Association (PSA) has warned the new Government was talking about having two prisoners to a cell as a way of squeezing more prisoners into the country's jails.
The prison population at OCF has fluctuated in recent months.
It reached the low 200s earlier this year, prompting management to close a wing which has since re-opened.
The department's southern assistant regional manager Ants Howie said double-bunk cells were used in prisons across the country and were common practice in other countries.
"Double-bunk cells are made use of the same as any other cell in New Zealand - when they are required. (They) are most frequently used when the prison population is high. Prison management use their custodial knowledge and experience to place prisoners together."
Incidents between prisoners sharing cells were infrequent and one or both prisoners were moved if staff become aware of any tensions.
Mr Howie also confirmed prisoners at some facilities, including OCF, had been locked in their cells for longer, a common practice at this time of year.
"Some prisons have recently taken up an 8am-5pm unlock system. This is not uncommon and has been used in previous years to allow staff to take some well-earned leave over the Christmas period."
But the PSA is concerned prison staff will be at greater risk because prisoners are being locked up for longer.
The PSA has 2000 members working for the department with 800 of those working in prisons.
"The Government is talking about having two prisoners a cell as a way to squeeze more prisoners into our jails. This will only increase tensions and heighten the risk of violence among prisoners and against staff," PSA national secretary Richard Wagstaff said.
He also said the National Government's law and order polices, which include longer sentences for crimes against children, no parole for repeat violent offenders and life without parole for the most serious murders, would also potentially cause trouble for staff.
"Prison officers fear that prisoners with no possibility of parole are more likely to lash out at fellow prisoners and prison staff because they'll feel they have nothing to lose."
Tensions in prisons were already high because a shortage of officers meant prisoners were spending longer locked in their cells.
