GB,
I believe the route is A216 and the volcano concerned is Manam.
Jetstar conducts its own assement on this route in real time, not a blanket avoidance. If the risk is increased, say orange, then Jetstar Aircraft will be re-planned.
As mentioned previously there are a number of airlines which adopt a similar appraoch to this route.
I urge you to think a little outside the QF square (no offence intended).