That first picture shows them at the Vietnam Airline counter. Vietnam Airline only has flights to Ho Chi Minh City and Hanoi from Sydney and Melbourne. It also has flights from Ho Chi Minh City to Siem Reap. Since the woman tweeting is from Sydney, I assume they left from there to Siem Reap via Ho Chi Minh City.
I found this pair of flights on the Vietnam Airlines website for March 16th, which I assume is similar to March 9th.
772 SYD 11:35 SGN 16:20 0
Sydney NSW , AU (SYD) - Ho Chi Minh City , VN (SGN)
Airbus A330-200, Vietnam Airlines
815 SGN 19:10 REP 20:10 0
Ho Chi Minh City , VN (SGN) - Siem Reap , KH (REP)
Airbus A321-100/200, Vietnam Airlines