Despite having lost their opening two games of the FIFA World Cup to Chile and the Netherlands, Australia have impressed many in Brazil with their proactive style and willingness to take on teams ranked much higher than them.
Under Postecoglou, who only took over as coach in October, Australia pushed hard for an equaliser 2-1 down against Chile only to lose 3-1, while they led the Netherlands 2-1 in the second half on Wednesday before succumbing 3-2.
Leckie, the 23-year-old forward based in Germany's second tier, conceded that he and his team-mates had never expected to do so well against two teams in FIFA's top 15.
"The whole team as individuals have all surprised ourselves a little bit," he said.
"For me, I definitely didn't think we would compete this well with the teams we have played.
"There have been times, especially the Holland game, we dominated areas of the game - to lose that game was very harsh.
"The way we have played … no-one expected it.
"I think the whole world expected us to come here and just put 10 men behind the ball and hope to pinch results.
"Although we haven't got the results, the way we have gone about it and the way we have played is something very exciting for the country."
Leckie has been key to Australia's strong performances in their first two matches of Group B, providing plenty of attacking threat and pace on the right wing.
The former Adelaide United winger has carved out a decent niche in the 2. Bundesliga, spending the past two seasons with FSV Frankfurt before signing for another second-tier club Ingolstadt just before the World Cup.
Leckie scored 17 goals and notched 14 assists in 63 appearances in all competitions for FSV.
While there has been speculation that Leckie could have secured a transfer to a bigger club after his performances in Brazil, the Melbourne-born attacker does not regret signing with Ingolstadt.
"I didn't know what was going to come at this World Cup," Leckie said.
"I always say that everything happens for a reason.
"So whether I could have maybe had other offers now after this World Cup if I hadn't signed, I never would have known."
Australia will complete their World Cup campaign against fellow winless side Spain on Monday.
