We work with the New Zealand Police Financial Intelligence Unit (FIU) and associated reporting entities like banks, casinos and international money transfer providers who identify suspicious activity, often involving cash.
Reporting entities have transaction monitoring systems and their staff are trained to identify activity that looks like it might indicate wrongdoing. When they are suspicious of a person or a transaction they have conducted, If the FIU thinks it could be tax evasion, they notify Inland Revenue. Examples of suspicious activity include withdrawing large amounts of cash on a regular cycle to pay staff, depositing money from a business transaction into a personal account, or having multiple personal accounts with different IRD numbers.
People sometimes think Inland Revenue won't find out about their cash transactions. But cash inevitably finds its way into the financial system, whether directly or by the people receiving the cash.