As Australia prepare for Tuesday night's clash to define the last four years, their mentor said it would be "flippant" to write off the over-achieving, under-resourced side that has recently regained some of its strongest squad members.

"I don't know why they would underestimate them, not sure what the basis of that is," Postecoglou said.

"They have strikers playing in some pretty strong leagues and scoring goals. There's certainly a resilience within the group and if you look at their results - they never got beaten by more than a goal.

"I think sometimes people are just a little bit flippant."

As reigning Asian champions, the Socceroos were widely expected to stroll to direct Russia 2018 qualification.

That they have been forced through the arduous play-off route says much about a lack of respect for continental foes and the challenges of gruelling travel and questionable conditions, according to Postecoglou.

But even back at home and set for a pristine ANZ Stadium pitch, to underrate an outfit that lifted in the second half last Thursday to seal a hugely controversial first-leg draw in Malaysia would be at the Socceroos' peril.

"They say we should beat everyone, but we've never had any evidence of that ever because it's just too difficult to do in this region," Postecoglou said.

"To play away from home in some of the places you play and just dispose of opposition easily just doesn't happen.

"Internally we are not underestimating the competition and the threats they pose, and that's why we make sure we focus on all the details they bring to the game.

"I think any reasonable person who knows football, I'd be very, very surprised if they think the level of opposition in Asia, particular in this round, is anything less than very, very competitive."