The below is an extract from the Qantas site, but if you take on over to expedia.ie you will experience significant savings between 100-200 Dollars
SYDNEY to
Singapore
$349 one way Economy
So Qantas has the flights to Bangkok on sales from Mel & Syd for $799 but when I checked expedia.ie those flights are starting from 476 Euro ($AUD600) ex Mel and 481 Euro ($AUD606) ex Syd.