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Unfortunately it's very difficult to predict what could happen. Not only is the timing and scale of a volcano eruption itself unpredictable, but a volcano's ash cloud can travel very far: in 2011, a number of airports in eastern Australia had to shut down because volcano in Chile erupted, even though it was 11,000 km away.
So if Mt Agung does blow, the impact on air traffic will depend greatly on wind patterns at the time. Long-distance flights should be able to reroute to avoid the island entirely quite easily, but flights to/from Bali and nearby destinations (Lombok, Surabaya) are likely to be heavily impacted.