Research by the Department of Corrections has found no link between double-bunking in prisons and assaults or disorder.
The research was carried out in the Northland, Spring Hill, Otago and the Auckland Region Women's prisons over the past two years. Forty 40 inmates and 40 prison officers were interviewed.
The study found that assaults and disorder incidents fell by 2% despite double bunking-being used in 70% of cells.
The Corrections Department says prisoners did not find the use of double bunking stressful, and adapted well.
But the president of the Corrections Association, Beven Hanlon, says he's more concerned now, following suicides in cells where bunks have been installed.
Mr Hanlon says he remains sceptical of the study's findings which the department has yet to release in full. He says the figures may look good, but the sample is small and the research coincides with a fall in prisoner numbers over the period.
The lobby group, the Howard League for Penal Reform, says it knows of at least two cases of sexual assault.