Paradoxically, in this case, heads out of the cockpit scanning visually may have delayed the Embraer crew becoming aware the conflict aircraft.
The PM on board ZPJ commented that during the visual circuit they changed the focus of their scan from inside the cockpit to outside the cockpit. They suspect that XGA was probably displayed as other traffic on their TCAS before they received the TA alert. However, there are no company procedures specific to the use of TCAS at non-controlled aerodromes.
https://www.atsb.gov.au/publications...r/ao-2016-106/
If the other aircraft did not have a transponder, the two aircraft may well of come a lot closer to each other.