I wouldn't be worried in the slightest about a 12 hour layover for your bags in Singapore. And with 4 or 5 daily flights SIN-SYD, in the unlikely event your bags are forgotten (almost a zero chance), they'll deliver the bag to your home a few hours later. You can always check at the gate, during boarding, to make sure your bags have made the flight with you. Just have your bag tags ready.