(1) Either you believe the threat is credible and you put it down at the nearest suitable airport in the least amount of time. Say Sydney at about 200km to your west, or FSP at 150km in the direction you're going (not a great fit, but doable). In both cases you could probably land within 20 minutes, a bit more if you aim for Gander (Fun history for that airport, great as an emergency diversion).
(2) or, you believe the threat is not credible. At this point you might as well continue the flight. Flying 90 minutes back does not seem (to me) to meaningfully reduce the risk if someone is actually planning to trigger a bomb anyway.