After a lacklustre start to the tournament with a 1-1 draw against unfancied Myanmar U-16, the Joeys needed a win at Phnom Penh's Olympic Stadium to kick-start their bid for top spot in Group A.
But things went from bad to worse for coach Tony Vidmar's side on Tuesday night as they went down 3-0 against Vietnam U-16, who also won their opening game 3-0 against Malaysia U-16.
The Vietnamese took an early lead, converting a penalty in the 17th minute, then doubled that advantage 10 minutes later with a well-taken goal on a fluid counter-attack.
It was still 2-0 at halftime, giving Vidmar plenty to discuss with his players.
However, it didn't help the young Australians find a way through the Vietnamese defence in the second half, and they conceded a third goal in the 77th minute after an unfortunate error from goalkeeper Nicholas Suman.
3-0 the final score as Vietnam U-16 go top of Group A and the situation turns dire for Australia U-16.
The Joeys face Philippines U-16 next, in a match they badly need to win.