The attacking midfielder once again missed out on a place in Lee Carsley’s squad.
The interim coach has not given Maddison a chance to show if he can be a star on the international stage.
Postecoglou told reporters: “Yeah look I am sure he is disappointed but at the same time he is playing well. You know, it’s not, sometimes it’s all you can do.
“These things come along at times, it's not always perfect in your career and there are situations and circumstances that mean you don’t always get what you want but it shouldn’t deter from or detract from everything else that you’re doing. So he is playing really well for us, I think he’s been outstanding all year.
“He’s making an impact in every game he plays, he’s a really important part of our midfield setup and if he continues to do that I am sure he’ll be selected again but he can’t just sort of put everything into one basket just being disappointed just because you’ve missed out. I’ve said that shouldn’t have any effect on him and the way he’s playing because he has been brilliant for us.”