Briefly, it goes like this. Augustine reads the story from John as an eschatological "mystery," so important that it has been saved for the final chapter of the book:
[I]nasmuch as there were seven disciples taking part in that fishing, Peter, and Thomas, and Nathaneal [sic], and the two sons of Zebedee, and two others whose names are withheld, they point, by their septenary number, to the end of time. For there is a revolution of all time in seven days. To this also pertains the statement, that when the morning was come, Jesus stood on the shore; for the shore likewise is the limit of the sea, and signifies therefore the end of the world. ...The story in Luke, he concludes, is of an earlier event. But both fishing trips are symbolic descriptions of the Church itself:
[J]ust as in [John] the Lord indicated by an outward action the kind of character the Church would have in the end of the world, so in the same way, by that other fishing [in Luke], He indicated its present character. In doing the one at the commencement of His preaching and this latter after His resurrection, He showed thereby in the former case that the capture of fishes signified the good and bad presently existing in the Church; but in the latter, the good only, whom it will contain everlastingly, when the resurrection of the dead shall have been completed in the end of this world.
... By these signs, and any others that may be found, [we see that in Luke] the Church was prefigured as it exists in this world, and [in John], as it shall be in the end of the world: the one accordingly took place before, and the other subsequently to the resurrection of the Lord; because there we were signified by Christ as called, and here as raised from the dead.The fish are Christians, the net is the Church itself. He contrasts the details of the two stories. In Luke, the net contains both saints and sinners. In John, it is saints alone:
[In Luke], the nets are not let down on the right side, that the good [people within the Church] alone might not be signified, nor on the left, lest the application should be limited to the bad; but without any reference to either side, He says, “Let down your nets for a draught,” that we may understand the good and bad as mingled together.
[But in John] He says, “Cast the net on the right side of the ship,” to signify those who stood on the right hand, the good alone. There the net was broken on account of the schisms that were meant to be signified; but here, as then there will be no more schisms in that supreme peace of the saints, the evangelist was entitled to say, “And for all they were so great,” that is, so large, “yet was not the net broken” ....You get the idea: the Church Militant is "by schisms torn asunder," the result of its catholicity. It includes the saved and the damned, heretics and troublemakers:
[In Luke], the multitude of fishes caught was so great, that the two vessels were filled and began to sink .... For whence exist in the Church the great evils under which we groan, save from the impossibility of withstanding the enormous multitude that, almost to the entire subversion of discipline, gain an entrance, with their morals so utterly at variance with the pathway of the saints?But the Church Triumphant is a different matter. There, the net is not torn apart, the Church is not divided, because only the elect are included:
[In John], however, they cast the net on the right side, “and now they were not able to draw it for the multitude of fishes.”
What is meant by the words, “Now they were not able to draw it,” but this, that those who belong to the resurrection of life, that is to say, to the right hand, and depart this life within the nets of the Christian name, will be made manifest only on the shore, in other words, when they shall rise from the dead at the end of the world?He goes on to add that St. John has specified the number of fish -- 153 -- because the number of the chosen is finite, and smaller than the total number of those who have been called. (There's quite a bit more about symbolic numbers, if you like that sort of thing; we don't.)
There is much to dislike about Augustine's interpretation. There is no room here for universal salvation, and scarcely any for grace. At least on the surface, it seems to presume double predestination, and even to admit a touch of works righteousness. God has called many and chosen a few; the reprobate -- those who are not chosen for salvation -- are distinguished both by their numbers and their "bad morals."
Frankly, those are not the hermeneutical maneuvers that we Lutherans, at least, so treasure about Augustine. It is Calvinist at best and Pelagian at worst.
But if we strip away the comparison with Luke, it is possible to find in Augustine a very useable interpretation of the Miraculous Draught in John. The image of the Church as a net, holding together a strange variety of individuals and yet not breaking, is quite beautiful. So too is the idea that it cannot be "drawn" -- not merely counted, but in a broader sense manipulated, swayed, pulled -- short of Heaven. (Needless to say, both of these are ideals; they do, inevitably, contrast with the Church as we know it.)
For a fine example of a modern sermon that draws heavily on Augustine's exegesis, we recommend Travis Poling's "Every Time the World Ends," in Brethren Life and Thought for Fall, 2006. If you're logged into ATLA, you can read it here.