The combined tax bills for Rio Tinto and BHP Billiton's move into Singapore marketing hubs a decade ago have now reached more than $1.9 billion, with further audits looming.

Rio Tinto's latest revelation that it is contesting tax assessments of $447 million for its Singapore operations from 2010 to 2013 comes on top of the $107 million it previously paid to settle claims covering 2005 to 2009, taking its total Singapore tax hit to $554 million.