I'm not so sure that is the correct information.
If you mean you have a boarding pass which you have printed yourself via the Jetstar website, I don't believe it is valid for travel out of Australia.
My experience has been that you still need to go to a counter to check in and Jetstar's website seems to confirm this:
'Please note that if you have a flight leaving from an international terminal you may only check-in via the airport check in counter.'
This is because a 'real person' has to look at and check you passport for validity for travel before a valid boarding pass is issued.
If this is the case, then you will need to clear customs and immigration and re check at the Jetstar counter.
I stand to be corrected on this as I'm not 100% sure.
I would contact Jetstar for clarification.