The authority of God and the Church can't be discussed: Augustine of Hippo is in the calendar of all Churches with an apostolic succession: Roman Catholics, Eastern Orthodox, Oriental Orthodox, and even Anglo-Catholics commemorate him as a saint in their calendars - what better testimony do you need? Or do you totally reject the authority of the Church?
I was not aware that he is actually an official Saint in the Orthodox Church. I've met people who think so but I've also met some to think he is a heretic for his teachings. I always thought he was to be referred to as Blessed Augustine rather than Saint.
In Orthodoxy, as Andrew rightly said, the classification blessed vs saint doesn't exist. The title "blessed" is associated to other saints. An example is st. Basil the Innocent - where the word for innocent is the same as blessed in Slavic. You surely know him very well: he is the Fool in Christ who was a real thorn in the flesh for tsar Ivan. Even in Catholicism, to tell the truth, associating the word "blessed" to a Christian doesn't mean that he isn't saint... the classification only expresses the extension and importance of such a person. Indeed, both blessed and saints are in heaven LOL No Catholic would ever question the holiness of Blessed John XXIII, Pope of Rome, despite we're still waiting for his canonization.
I think that in this case the most reliable sources, despite all disputes, for the Eastern Orthodox Church are the quotations of Augustine among the Church Fathers in the Fifth and Sixth Ecumenical Councils, and the importance of his role in condemning Pelagius and Celestius, whose heresies were condemned both in the Council of Carthage (of an ecumenical value through the Trullan Synod) and the Third Ecumenical Council.
Here's a specific quote from http://orthodoxwiki.org/Augustine_of_Hippo
The Fifth Ecumenical Council, held in Constantinople in A.D. 553, listed Augustine among other Fathers of the Church, though there is no unqualified endorsement of his theology mentioned (just as there is none for most saints of the Church):
We further declare that we hold fast to the decrees of the four Councils, and in every way follow the holy Fathers, Athanasius, Hilary, Basil, Gregory the Theologian, Gregory of Nyssa, Ambrose, Theophilus, John (Chrysostom) of Constantinople, Cyril, Augustine, Proclus, Leo and their writings on the true faith. (emphasis added)
In the acts of the Sixth Ecumenical Council (not yet translated into English), he is called the "most excellent and blessed Augustine" and is referred to as "the most wise teacher." In the Comnenian Council of Constantinople in 1166 he is referred to as "Ό Αγίος Αυγουστίνος - "Saint Augustine."
In Christ, Alex