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1 hour 20 min is a very long connection time in SIN. You'll get bored waiting. Only potential issue I see is that you are taking the last SIN-HKG flight of the day, so if the SYD-SIN flight is massively delayed, you'll need to spend a night in SIN. It's unlikely, but the possibility is there.
I think you should be OK. I have managed to make ~50min-55min connections in SIN without hassle.
Have you checked the history of the SYD-SIN flight?