Hi Trace,
Actually, it's a little more complicated now since they changed the regulations in March of this year. Check out the Singapore Airlines and Australina Government links below:
Singapore Airlines
Duty Free
From the first site it looks as though you can buy duty free in transit in Singapore BUT they must either be:
1) purchased on board the plane
2) be delivered by duty free staff to the gate in a sealed bag
Frankly, IMHO it's a bit over the top, seeing as you're buying it in transit and not in the city but those are the rules....The second site seems to say that it's only liquids and aerosols bought in transit that you can't bring through, which makes a bit more sense.
Hope this helps!