The Victory are winless after their 2-1 opening-round loss to the Melbourne Heart.
Midfielder Thomas Broich opened the scoring in the 22nd minute and Eric Paartalu made it 2-0 only a minute later as the Victory were strained by the absence of Archie Thompson, Mark Milligan and Marco Rojas who were on international duty.
Besart Berisha scored in the 64th and Mitch Nichols in the 82nd before Albania international Berisha completed the rout in stoppage time.
Postecoglou later played down his former Brisbane connection.
Brisbane is "not my team. It never has been," he said. "To me it's just another game.
"I am coaching the Victory club. My responsibility lies solely with getting this club back to where it belongs and performing like our supporters think they deserve - that's our focus."
On Sunday, the Wellington Phoenix overcame the loss of seven players and their coach to New Zealand's World Cup qualifying campaign by snatching a 1-1 draw with the Melbourne Heart. Wellington has yet to win in seven A-League matches in Melbourne against the Heart and Victory but added a fifth draw to seven losses.
The Heart were reduced to 10 players early in the second half when Simon Colosimo was sent off for deliberately handling a goal-bound shot.
Later, Central Coast beat Perth 1-0 with a 69th-minute goal to Daniel McBreen.