Edit: I won't say you should be fine given the comments below mine anymore. But my times should be accurate. You will have to decide if that's enough time for you.

Immigration is 5-45 min at worst, if more than 20 min your bag should already be through anyway. Then 5 min walk to the domestic terminal. So even if the immigration queues are really long, you should still be at the Jetstar counter about 45-60 min before your flight.

The caveat of course is if your BI flight is late.