IIRC (its been a little while since I last visited Singapore) if you clear customs, you are only allowed one open pack.

If you just transit w/o clearing customs there is no limit I'm aware of. If you are in transit or leaving Singapore you can buy cartons duty free for cheap at Changi duty free shops, but you cannot buy duty free before entering Singapore.
If you don't declare.... the fine is very expensive if you get caught. If you are staying in Singapore for a few days before heading home probably best to buy in Changi duty free shop before departing. Probably 2-3 times the price of cigarettes in Indonesia but still probably WAY cheaper than Australia.
Edited.... re read your post.
"Flying DPS SIN for five days then CGK SYD"
Buy at CGK