The Government is to amend the existing offshore banking unit regime to make dealings with related parties, including the transfer of transactions between an OBU and a related domestic bank, ineligible for OBU treatment.The closure of loopholes in the OBU regime, announced in last night's Budget, is part of a group of measures the federal government has announced to protect the corporate tax base. Transactions between OBUs, including between unrelated OBUs, will be ineligible for OBU treatment. More than one hundred OBUs are registered.The Government expects the OBU measures to add A$320 million to revenues over the forward estimates period."It appears this reform has been put into place to target transactions that are similar to the 'asymmetric swaps' of several years ago," Patrick Broughan, a tax partner at Deloitte said."These were essentially swaps that an OBU, the domestic bank and an offshore institution entered into to hedge certain currencies or indexes."Broughan said that in 2010 the Tax Commissioner issued a determination that effectively put an end to asymmetric swap types of arrangement and that the budget announcement seems to effectively legislate the Commissioner's 2010 determination.  The Australian Financial Markets Association griped in a media release that "this may impact existing bona fide business arrangements, requiring remedial action to occur in an extremely short time-frame in order for continued compliance with the regime."The Government plan to protect the corporate tax base also includes increasing Australian Taxation Office compliance checks on offshore marketing hubs and business restructures, and preventing sophisticated investors from dividend washing. The Government will also address aggressive tax structures that seek to shift profits by artificially loading debt into Australia. This will involve tightening the thin capitalisation rules and increasing the de minimis threshold from $250,000 to $2 million of debt deductions.The Government estimates these measures will save more than A$4 billion over the forward estimates period.