Vicente Del Bosque's side were sent crashing out of the World Cup against Chile last time out, with the Socceroos looking to add to their woes
Both teams are already eliminated from the tournament having lost their opening games against the Netherlands and Chile.
While Spain have been disappointing, however, Australia have impressed and are unfortunate to miss out following spirited performances in Brazil.
“We would like to win tomorrow,” Postecoglou said, speaking in a press conference on Sunday. “This is the moment to convert ourselves to what we want to be.
“For me this is not a nonsense match, we want to win. Tomorrow I’ll choose a competitive team to let us win. That’s what we want to achieve.
“Our objective is to offer the best performance but let’s not forget that we are facing the current champions of the world.”
Postecoglou refused to be drawn on la Roja’s failings in the tournament to date, but did acknowledge the high standard of Group B.
“I think everybody had an opinion about Spain and lots of people thought Spain would be the favourites to win the World Cup,” he began.
“Group B has been very hard, Chile and Netherlands have done very well. I've seen Spain's matches and a lot of times the prediction is that Spain was going to win, but when they faced Netherlands they missed chances and it changed their tournament.
“But this is the World Cup. Spain are still the champions of the world, I have a lot of respect for them, they have a team made by the current champions of their clubs and Champions League.
“When we arrived at this tournament we knew that we were going to play against the best of the world. We are expecting that Spain will play very well tomorrow, they are a strong team.”
The former Melbourne Victory coach was also keen to consider Australia’s showing this summer within the context of the Asian federation’s performance as a whole.
“For our confederation the next challenge is to have an impact at the World Cup.
“Asian teams are having a good performance, South Korea are back after a regeneration period, Japan are a team with a lot of quality, but this time they haven't achieved the results, this is the greatest challenge.
“It is our responsibility for the Australian team to offer a better performance against Spain.”