Australia will depart Brazil having failed to take a point from a difficult Group B, and were soundly beaten 3-0 by Vicente del Bosque's men in Curitiba on Monday.
Despite finishing bottom of their pool, Australia have won many admirers for the attacking nature of their performances.
Head coach Postecoglou admitted that the efforts exerted in losses to the Netherlands and Chile had caught up with his team, but feels they have made significant improvements.
"It was disappointing. They (Spain) dominated after the first 15 minutes and we struggled to get into the game," Postecoglou told reporters.
"There were a few tired legs out there (for Australia). They had put a lot of effort in the previous two games and it probably just caught up with them.
"As for the reaction to the loss, people can think what they want but there is no way this result affects what we have done here."
Postecoglou has now set his sights on Russia, where the next World Cup will be held in 2018, and believes that tournament will be a key marker to test the progress his young side have made.
"We've certainly got some positives out of it and stuff we need to work on," he added on SBS.
"We wanted to come to this World Cup and try to take it to three absolute world-class teams. I think we've done that. What we've found out is we're not at that quality yet.
"In four years' time, if we keep having that sort of intent, I think we'll bridge the gap and hopefully come back and be the team that's more dominant than the opposition."
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