[I've read several places where St. Augustine speaks of the Church praying and making offerings for the departed. However, he does not mention them as suffering in Purgatory.
Here are three quotes from widely differing centuries (5th, 17th and 20th) which show the same unanimous teaching on life after death...
The teaching of Saint Augustine of Hippo:
"During the time, moreover, which intervenes between a man's death
and the final resurrection, the soul dwells in a hidden retreat, where it
enjoys rest or suffers affliction just in proportion to the merit it has
earned by the life which it led on earth."
Augustine, Enchiridion, 1099 (A.D. 421).
The 1980 Resolution of the ROCA Synod of bishops on the toll house belief...
"Taking all of the foregoing into consideration, the Synod of Bishops resolve:
In the deliberations on life after death one must in general keep in mind
that it has not pleased the Lord to reveal to us very much aside from
the fact that the degree of a soul's blessedness depends on how much
a man's life on the earth has been truly Christian, and the degree of
a man's posthumous suffering depends upon the degree of sinfulness.
To add conjectures to the little that the Lord has been pleased to reveal
to us is not beneficial to our salvation..."
Interestingly enough, this is almost a word for word repetition of what Saint Augustine said 1500 years earlier!
The Synod of Constantinople of 1672:
"We believe that the souls of the departed are in either repose or torment
as each one has wrought, for immediately after the separation from the body
they are pronounced either in bliss or in suffering and sorrows, yet we
confess that neither their joy nor their condemnation are yet complete.
After the general resurrection, when the soul is reunited with the body,
each one will receive the full measure of joy or condemnation due to him
for the way in which he conducted himself, whether well or ill."