Since we wanted to get the earliest flights possible after the prices came down after peak season, we thought we'd better book early.
The travel agent was about $700 more expensive than booking directly with Singapore Airlines, and I was surprised that our tickets from Melbourne-Singapore-Siem Reap/Phuket-Singapore-Melbourne were only slightly more expensive than a Melbourne-Singapore return ticket.
We have 1 night in Singapore on the way out and 2 nights on the way back. We didn't have to do this, but wanted to break the journey and not have to wait around in airports for connecting flights.
See the 'Where we hope to go' photo gallery to see our tentative route.