A crackdown on tradies' cash jobs is yielding "outstanding" results, says the tax department.

Inland Revenue has launched an advertising campaign targeting cash jobs in a bid to recoup unpaid income tax from the country's multimillion dollar 'hidden economy'.

Sub-contractors in north and south Auckland - specifically Flatbush, Takanini, Silverdale and Albany - were being targeted in the 'declare it all or risk everything' campaign, which will run until the end of June.

Read more: IRD chases down tradies' cashies

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Around a week into the campaign, the IRD has praised its effectiveness and say more tradies are coming forward to declare income.

Inland Revenue's marketing and communications group manager Andrew Stott said the results of the campaign so had been "outstanding".

"It's early days but already we're seeing a shift in attitude. What's also clear is there's a common misconception we only target 'small fry' evaders - this is not the case.

We have identified tradespeople as being a high risk sector but we take all levels of tax evasion seriously. We're finding people who receive $30,000 in cash for a job and put it straight into their pocket.

"Our concern is when individuals or businesses don't pay their fair share of tax they're basically cheating those who are doing the right thing and expecting others to pay their way," he said.