Can someone please give me a possible reason why he was told not to return to Poland?
Good to see another Aussie here. As Katheryn has touched on, after WW2 Poland was not really Poland anymore. It was controlled by the Soviets, and if you're aware of what the Soviet Union was like under Stalin, then you'll have an understanding of what it must have been like for Poles.
The simple answer is that he would have been told not to return because where he came from was under Soviet rule and if he returned he may have been imprisoned or shot. Poland was a wasteland too. I hope your dziadek had a happy life in Australia, as did many Poles who came here and forged new lives after WW2.
Perhaps it might be an idea to visit a local Polish Club? Whilst they're fast dying out, there may be a WW2 veteran there who would happily answer your questions. Cheers