The head of Corrections has defended cutting staff in 2015, despite efforts now to recruit another 800 before the end of this year.
An email from the national commissioner at Corrections - which was made public in December - said the department would need 800 extra staff by September.
Chief executive Ray Smith was today questioned by MPs at a Parliamentary select committee about the move to axe almost 200 jobs in 2015.
Mr Smith told them the prison population had surged to more than 10,000, compared to about 8500 three years ago.
He said no-one could have reasonably predicted that increase.
"That rise has mostly occurred within the last 24 months and probably accelerated in the last six. So, the fact that we lost some people along the way because we thought we were getting smaller, I think, was a reasonable decision to make at the time."
Mr Smith said the growth rate in the past two years had been the highest in three decades.