Neill: I respect Ange's decision
Neill, who signed a deal with Championship outfit Watford until the end of the season, said fitness was not an issue and Postecoglou had left the door open for the veteran defender to make his third World Cup appearance.
The 35-year-old’s last competitive match was in November for J. League side Omiya Ardija.
A key figure in the Socceroos so-called Golden Generation, Neill said he respected Postecoglou’s decision to overlook several senior players while he tried a “few ideas”.
"From my point of view it's not a fitness thing - I've been training for six weeks now," Neill told BBC Sport.
"There's a few players in the Australia squad who are in leagues such as the MLS or in China which haven't started yet.
"Rhys Williams (Achilles injury) at Middlesbrough has left a big hole in our defence and we have a lot of untried and untested players.
"The manager has this game only to work out what sort of defenders he is going to take to the World Cup and he's probably seen enough of me to know what I can and can't do.
"The message to me was to get used to the environment at Watford and push for a position as soon as possible."
He told the Watford Observer Postecoglou had discussed his reasons for leaving him out of the Ecuador friendly.
"Yes he did explain the decision," Neill said. "There are three or four experienced players not in the squad because it is the last chance for the squad to get together before May and the manager wants to try a few ideas and I respect his decision.
"My focus now is on making sure all the reports he reads about me and the games he sees of me will be positive ones. I have been training hard and I hope to play in as many games as I can."
He added: "It is definitely my last chance to compete in a World Cup and we have a really challenging and exciting group against Chile, Holland and Spain.
"For any player, going to a World Cup is amazing but to do it three times will be something I would never forget and an opportunity I don't want to let go. That is one of many reasons why I am highly motivated."
Neill said a desire to continue playing football at a high level and see more of his family was behind his return to England where he spent 14 years lining up for the likes of Millwall, Blackburn Rovers, West Ham and Everton.
He spoke positively of his time in the J.League and UAE – and the standard of football in both leagues – confirming his experience and leadership qualities had helped seal the deal with Watford.
All going well the defender hasn’t ruled out extending his stay at Vicarage Road.
“I want to play football at a high level for as long as possible and be challenged and tested,” he said.
“I know I will be comfortable here in a strong London club but it all depends on performances and results. In the next two months, I think my performances and where the club finishes will dictate what happens next year.
"Looking forward on a positive note, I would love to be a part of next year and let's hope it is in a team that is possibly in the play-offs and then who knows where they will be next year."