Corrections Regulations 2005, reg 179
If you’re under 18 you’ll usually be placed in a “youth unit”, with other prisoners who are under 18. If your prison doesn’t have a youth unit or if there’s no bed available in the youth unit, you may be placed in a segregated adult unit. This may also be because the prison is still assessing exactly what youth unit you’re going to be placed in – this could take one or two weeks.