Get a Linux liveCD and use it to flash with Nokia firmware, maybe?

I assume you already searched for singtel here; there's 5 results at the moment, and one of them includes a discussion of success using the Linux flasher, so it's probably worth the effort to get a bootable Linux CD/USB drive.

Another option (but not one anyone's reported success with, AFIAK) is to rewrite the versioning information in the config partition; allusions to this were made in that thread, and it's possible that setting the version to some legitimate non-singtel version permits an update from Windows. Then again, fiddling with an undocumented config partition includes substantial chance of breaking things so bad you would need to reflash (from Linux) anyway...