Inland Revenue is cracking down on tradies who aren't paying tax.

It's launching a campaign to make tradespeople aware of their obligations, particularly when it comes to cash jobs over $20,000.

Marketing manager Andrew Stott said a recent survey revealed more than 10% of Auckland tradies know of jobs of this size done under-the-table.

"Most New Zealanders pay their tax, believe it's the right thing to do because it pays for so many of the things that make New Zealand a great place to live. But there are some who don't want to pay tax on all their income and they need to know that that is not ok and we won't let them get away with it."

Mr Stott said they're really just trying to help people get it right, through an easy voluntary disclosure process.

"If they don't do that then when we find them, and nowadays it really is a when, then there are significant financial penalties and potentially some criminal action."