IRD are continuing to focus on cash jobs among builders and other tradespeople. They are looking into record keeping and tax compliance in these industries with the most recent focus being on Christchurch. It’s never too late to voluntary disclose income that might not have been declared previously in your tax returns. A voluntary disclosure to IRD will generally reduce the amount of short fall penalties applied, especially if this is before IRD notifies you of a pending tax audit.
If you are paying cash for a job, keep in mind that you then have no record of the work being done in the event of something going wrong.