Augustine was clearly amillennial. He certainly was not postmillennial (although postmillennialists use him to support their viewpoint.) He was born in 354 A.D. and wrote after 386 A.D.
A few key points from his writings...
Book 20 — Concerning The Last Judgment, And The Declarations Regarding It In The Old And New Testaments.
Chapter 7 — What Is Written In The Revelation Of John Regarding The Two Resurrections, And The Thousand Years, And What May Reasonably Be Held On These Points.
The evangelist John has spoken of these two resurrections in the book which is called the Apocalypse, but in such a way that some Christians do not understand the first of the two, and so construe the passage into ridiculous fancies. For the Apostle John says in the foresaid book, "And I saw an angel come down from heaven. . . Blessed and holy is he that hath part in the first resurrection: on such the second death hath no power; but they shall be priests of God and of Christ, and shall reign with Him a thousand years."
Those who, on the strength of this passage, have suspected that the first resurrection is future and bodily, have been moved, among other things, specially by the number of a thousand years, as if it were a fit thing that the saints should thus enjoy a kind of Sabbath-rest during that period, a holy leisure after the labors of the six thousand yearssince man was created, and was on account of his great sin dismissed from the blessedness of paradise into the woes of this mortal life, so that thus, as it is written, "One day is with the Lord as a thousand years, and a thousand years as one day," there should follow on the completion of six thousand years, as of six days, a kind of seventh-day Sabbath in the succeeding thousand years; and that it is for this purpose the saints rise, viz., to celebrate this Sabbath.
And, this opinion would not be objectionable, if it were believed that the joys of the saints in that Sabbath shall be spiritual, and consequent on the presence of God; for I myself, too, once held this opinion. But, as they assert that those who then rise again shall enjoy the leisure of immoderate carnal banquets, furnished with an amount of meat and drink such as not only to shock the feeling of the temperate, but even to surpass the measure of credulity itself, such assertions can be believed only by the carnal.
They who do believe them are called by the spiritual Chiliasts, which we may literally reproduce by the name Millenarians. It were a tedious process to refute these opinions point by point: we prefer proceeding to show how that passage of Scripture should be understood.
The Lord Jesus Christ Himself says, "No man can enter into a strong man's house, and Spoil his goods, except he first bind the strong man" — meaning by the strong man the devil, because he had power to take captive the human race; and meaning by his goods which he was to take, those who had been held by the devil in divers sins and iniquities, but were to become believers in Himself.
It was then for the binding of this strong one that the apostle saw in the Apocalypse "an angel coming down from heaven, having the key of the abyss, and a chain in his hand. And he laid hold," he says, "on the dragon, that old serpent, which is called the devil and Satan, and bound him a thousand years," — that is, bridled and restrained his power so that he could not seduce and gain possession of those who were to be freed.
Now the thousand years may be understood in two ways, so far as occurs to me: either because these things happen in the sixth thousand of years or sixth millennium (the latter part of which is now passing), as if during the sixth day, which is to be followed by a Sabbath which has no evening, the endless rest of the saints, so that, speaking of a part under the name of the whole, he calls the last part of the millennium — the part, that is, which had yet to expire before the end of the world — a thousand years; or he used the thousand years as an equivalent for the whole duration of this world, employing the number of perfection to mark the fullness of time.
But note that the first option is false since this time period is now over.
Chapter 9 — What The Reign Of The Saints With Christ For A Thousand Years Is, And How It Differs From The Eternal Kingdom.
But while the devil is bound, the saints reign with Christ during the same thousand years, understood in the same way, that is, of the time of His first coming. For, leaving out of account that kingdom concerning which He shall say in the end, "Come, ye blessed of my Father, take possession of the kingdom prepared for you," the Church could not now be called His kingdom or the kingdom of heaven unless His saints were even now reigning with Him, though in another and far different way; for to His saints He says, "Lo, I am with you always, even to the end of the world."