Jetstar "international" flights from Darwin to domestic destinations say 60 minute check-in some places and 30 minute check-in other places. Like OP, I thought it was a bit rough if you took note of the 30 minute time when it was actually 60.
However, these flights (at least JQ82) actually leave from the domestic part of the terminal in Darwin and have a 30 minute check-in. Maybe it's the same in Cairns?
All JQ "international flights" ie
JQ82 DPS/DRW/BNE
JQ62 SIN/DRW/MEL
JQ74 SGN/DRW/SYD &
JQ76 MNL/DRW/SYD cease being international when they arrive in DRW as all pax disembark, clear customs & then have to bag drop their luggage at the JQ checkin counters.
All pax then head back upstairs & reboard the same aircraft however from the domestic side of the airport ie they don't go through customs again even as a domestic pax & just mingle with all the domestic joining pax in DRW.
The continuation of the above flights is purely on a domestic basis ie you depart from the DRW domestic terminal & arrive at the domestic terminals in BNE/MEL/SYD. You would purchase any duty free on arrival in DRW which you are permitted to carry on the next sector as it's a domestic flight.
These above flights are like any other JQ domestic with regards to baggage close off being at T-30. Again I would highlight arriving at the airport one hour before just to give yourself a buffer in case of any number of unexpected things that could happen.
The only exception in
JQ58 SIN/DRW/CNS where you remain in transit airside in DRW if continuing on to CNS where you clear customs. Do not purchase duty free until arrival in CNS otherwise it will be confiscated in DRW when you disembark & go through the screening point into the transit area.
For domestic joining pax DRW/CNS as this is still an international flight ie you would go through customs DRW as a domestic & arrive at CNS intl terminal where you again go through customs on arrival.
I understand the cut-off is T-45 for JQ58 DRW/CNS so you'd want to arrive at least 60 minutes prior at the latest. I think that is a local rule in DRW as I understand that flying CNS/DRW on JQ57 the flight closes at T-60 so you'd be wanting to arrive there one hour and a half prior to departure. I believe most other international flights where you are travelling domestically have the same T-60 cut off ie get there no later than T-90!
Many people in CNS catching the flight arrive by mistake at the domestic terminal so if you have given yourself a buffer you would still have time to make it on foot to the domestic terminal without missing your flight.
Some pax just like to stand their and argue that they weren't told it was an international flight, not their fault etc etc but ultimately they will still miss that flight so isn't it better to err on the side of caution & give yourself a bit of extra time???