The time between take off and a last transmission were close to 19hrs and she had the chance up until then to report she was heading back to Rabaul but never mentioned anything was wrong. Radio seemed to work until then as she made a call to the Itasca "We must be on you but cannot see you but gas is running low" This seems likely she was within the area.
Even if the figures in the flight manual on fuel are high and she learns it right out she would still not even make it half way back to New Britain.
Take off 4180lts on board
Fuel Flow 150lts/HR gives her close to 28hrs Endurance
By the time she got to Howlong Island she had maybe and this is generous 9hrs left and it's more likely she had even less fuel then this on board as she transmitted fuel is low.
To say the wreck is in New Britain really means you need to ignore what she transmitted and she did a 180deg change of heading without even knowing it.