There is a "free" bus which is funded by some airlines but BNE is a lot less convenient for this connection than say MEL or even SYD. Hopefully the airfare was lots cheaper to make up for the inconvenience. QF intl/domestic does include a free transfer (bus running landside between the 2 terminals), not sure whether Etihad also provides this or QF on separate ticket. There is a fair bit of walking landside at both terminals (probably longer than the airside part, which can be assisted, so not too useful). I'd say 120 min is the minimum connecting time, so on separate tickets would want at least 180 min.