The Australian Tax Office is meant to be one of the most secure and forensic agencies in the country - we trust it to get things right. 

But its vulnerabilities were exposed, after a criminal syndicate managed to steal more than $100 million of taxpayers' money. 

Today, Four Corners reporter, Paul Farrell, takes us inside the biggest tax fraud in our history, and explains how it was exposed. 

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Paul Farrell, investigative reporter, Four Corners, ABC TV and ABC iview