Hi VinodhK7,
I think I found it... Go to Tax > Setup > Foreign trade > Foreign trade parameters. Set focus on the Country/Region properties.
On this page, you can setup the type of each country. However, the global country list is provided, the details here are empty. When there is no record, it will return the default enumeration for a country which is Domestic.
You can save yourself from a customization by setting AUS as domestic and SGP as Third country. This is a setup per legal entity.