The Corrections Department is blaming its need to recruit hundreds more staff on changes to bail laws and tougher sentencing.
Last year, Corrections axed 200 jobs and closed several prison units, saying a cut in reoffending rates meant fewer prison staff were needed.
But in September, Corrections said it needed another 600 staff to cope with a growing prison population.
And an internal email shows that figure has now risen to 800.
Corrections' chief custodial officer, Neil Beales, says predicting prison numbers is an inexact science.
"One of these things you can never predict is changes in legislation, what they are going to bring in terms of how that legislation is then called into action," Mr Beales said.
"That, couple with changes to the bailout... What we've seen is a rise in the remand numbers and that rise continues, and we've seen people are now staying in prison for longer and doing longer sentences."