Information obtained by New Zealand's National Party revealed that 193 of the machines were installed at 16 locations across the country after a record-breaking heatwave last summer, at a reported cost of NZ$1,095,308 ($730,000).
National Party leader Simon Bridges called the move "an extraordinary waste of taxpayers' money," according to The Guardian.
"Government departments under Labour have got the idea that taxpayer money is there to spray around on whatever," he said.
Department of Corrections officials, however, defended the decision to bring in the machines, which prisoners do not have access to.
The department's acting national commissioner, Andy Milne, said the 2017-2018 summer heatwave caused substantial discomfort among staff and risked raising prisoner tensions.
“The situation had the potential for significantly increased prisoner tension and aggression, and the real risk that prisoners or staff would be seriously hurt as a result,” Milne said.
The department said the slushy machines, along with other cooling measures like wet face cloths and personal fans, have been a success because there have been no major incidents between prison staff and inmates during the summer, despite high temperatures.
January 2018 was New Zealand's warmest month in any reliable temperature records dating back to 1867, with highs around 100 degrees Farenheit.