The Australian Taxation Office is reminding taxpayers that they have just five days left to declare any hidden offshore taxable income, before facing tougher penalties.
The ATO's long-running voluntary disclosure initiative will close on June 30, the department said.
Under the initiative, which has been running since July 2007, people are able to disclose taxable hidden income in exchange for reduced penalties.
The scheme has netted around $73 million in tax revenue from the disclosure around $406 million in hidden income during its lifespan, the ATO said.
For taxable income disclosed before the offer expires, only additional taxable income of more than $20,000 will need to pay a shortfall penalty of 10%.
But if taxpayers are caught with extra income after the offer ends, they face penalties of up to 90% of the omitted income, and more serious cases will be prosecuted.
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