A federal Treasury paper warns that changes to offshore banking unit (OBU) laws could attract opposition from international regulators and also cut the tax take.The paper, available from the Taxation Institute of Australia, was delivered to a Taxation Institute conference last year by William Potts, manager of the taxation of financial arrangements unit in Treasury's business tax division.The Johnson Report, as it is called, is on Australia's role as a financial centre and recommends making the laws easier to use. Treasury, however, is concerned the OBU laws' concessional 10 per cent tax rate could be used to reduce tax in areas the laws aren't intended to cover.