Tradies are being reminded that every undeclared cash job leaves a trail.

Inland Revenue's launched a campaign to warn consumers and businesses about the risks of doing 'cashies'.

Messages will start appearing in Auckland, Wellington and Christchurch this week, to address tax crime in the building sector.

IRD spokesman Andrew Stott said things can be easily uncovered when looking for irregularities in a tradie's books.

"When cash jobs are been done people have thought 'oh Inland Revenue must be blind to that' but we're not, we can see when cash is spent and that is a big part of the way that we pick up when things are going wrong."

Stott said undeclared cash jobs are not only a risk to businesses, but also the customer.

"If you do a cash job you don't have a proper receipt probably and then you don;t have a recourse to law if the job is shoddy."