I've done this a few times from Sydney with SQ. You are running the risk that if the first flight is delayed and you miss your second leg then you will need to purchase the second leg again at last minute prices. I always allow as much time as I can between the two flights, preferably overnight. That adds accommodation cost but does give you an excuse for a night in Singapore.

At Sydney give the check in agent details for both legs and ask them to check you in all the way through. They always do this for me.