At the b team, the 737-800 with winglets had a comfortable endurance of about 6.5 hrs with full tanks (approx 20.5 tonnes). To carry that fuel though you would have to cap pax at about 150~160 (and nil freight of course) due max structural weight. So could do MEL/SYD/BNE to Bali without too much drama (always seemed to be able to carry 60 mins hold if required) and of course PER - HKT was done at between 6 and 6.5 hours. Classic A320s
AFAIK have a couple of different tank options but understand the Jetstar 320s are mostly 18T fuel ? Happy to be corrected.