To restrict results to a specific country use the component-filtering.
The url should be http://maps.googleapis.com/maps/api/geocode/json?address=Singapore%20505468&sensor=false&components=country:SG
But that's not the issue here, the geocoder-result is wrong, because the result has the country set to SG
, but the location is wrong(placed in india).
I'm afraid with the given address(it appears that the postcode doesn't exists), the only thing you can do is to report the wrong result to google.