With 4 ATR`s by July i`d hazard a guess they`ll be replacing some of the E170 shorter hops on a one-for-one basis.
From Flightglobal:
"The ATR will form the foundation of our regional network plans, with the first six ATRs replacing our current Embraer E170 fleet and the additional aircraft flying to new regional destinations," Borghetti says.
Virgin Blue's new ATR72s to arrive from May