Thanks for your reply. Singapore Airlines mostly have "for-two" offers; i.e. offers for two persons or more. Fares for single persons are a lot higher, around SGD$300 and above the sale fares. Malaysia Airlines had cheaper fares compared to Singapore Airlines by 20-30%.

The thing is Garuda Indonesia, Delta partner, only have VERY expensive fares EX-SIN southward bound to Australia. e.g. SIN - PER SGD$840. The same route on Qantas or Malaysia Airlines costs around $600.