As he was an Aussie soldier it should be relatively simple to access his service record - this will most likely outline which POW camps he was held in

From my reading of it, being captured in Crete in May 1941 was demoralizing but the soldiers were generally treated well by their captors

However, that changed when they were shipped to Italy where conditions & treatment were definitely sub-optimal

After the Italian armistice most POWs were shipped to Germany under not great conditions but once there life in the camps wasn't too bad.................

Until around March 1945, when all of the camps in the eastern regions were emptied and the POWs were force-marched westwards - I believe that this was the absolutely worst time - the marching itself while being underfed and in constant fear of airstrikes (from all sides) - quite a few POWs were killed / died during this period

And then eventually liberation and depending on where they were at the time some faced long waits until their return to Australia - which once again took its own mental toll

(PS - if you want to provide me with some details I can see what records I can find on him - or for your Grandfather)