"He has got to play regular first team football now he is in England and we couldn't guarantee him that," says the Hammers boss.
"With the experience he had already gained playing in the J-League in Japan, playing in the reserves and then maybe half a dozen games a year in our first team wasn't going to benefit his game.
"I am sure he will want to get back in the Australian team and to do that he needs to get regular first team football.
"It was only right that when Portsmouth came in we allowed him to go."
Glenn reveals that the deal took some time to negotiate, adding: "It was a very long, drawn out transfer which went on for the best part of a month, but eventually all parties were agreed.
"He went with my best wishes and those of the club. He was a pleasant enough man to deal with who worked hard in training, and hopefully he will be a big success in Portsmouth.
"Another factor in allowing Hayden Foxe to go was that we have got Ian Pearce fit again. He is very versatile, equally at home at the centre as he is at right full back, and now he is fit we want him to stay that way because he is going to figure in my plans in a big way next year."
As well as the return of Ian, Glenn points to the upsurge in form of Christian Dailly as being another factor in his decision.
"I think it is fair to say that Christian didn't do so well when he first came to the club, and he just got better and better through the season.
"They have done very well, Tomas and Christian, who was consistent and safe, which is all you ask for from your defenders.
"I feel that Tomas Repka has, without doubt, had a big bearing on that, and at times last year they looked a very good partnership.
"The one thing both of them have got is pace, they are strong players, and decent in the air considering how they are both just six foot - the partnership was a decent one."