Inland Revenue's cracking down on unpaid tax bills.
It's been allocated an extra $35 million in Budget 25 to boost its tax compliance and collection activities.
The tax department expects to return an additional $4 for every dollar in the first year, and $8 in year two.
IRD Commissioner Peter Mersi told Mike Hosking it's hard to estimate how much tax is owed across the board.
He says they don't really know the size of the gap, but believes it's around $9 billion.
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The Mike Hosking Breakfast