The Plumbers, Gasfitters and Drainlayers Board is set to crack down on rogue plumbers.
The board says it will investigate one North Island region and another in the South Island within the next two months, following a series of complaints and tip-offs about people working illegally in the trade.
A similar exercise by the board in 2010 in Northland netted 10 unlicenced plumbers in business.
Board deputy registrar Kern U'ren says prosecutions are possible this time, as tradespeople are continuing to flout the law.
Those who have not renewed their licence will be investigated and anyone found breaking the rules could face a maximum $10,000 fine.
Mr U'ren says two inspectors will be on the road to check exactly what work tradespeople are carrying out.
He is not revealing which regions will be investigated.