Raids on illegal plumbers have netted 25 people who will be prosecuted in court or penalised by the industry regulator.
Investigators from the Plumbers, Gasfitters and Drainlayers Board swooped on the Auckland suburbs Millwater and Flatbush late last year.
Sixteen illegal operators who are not qualified to carry out plumbing work were found in Millwater and nine in Flatbush.
The regulator said it would target more places identified by callers to its report-a-cowboy line.
It has warned householders to ask plumbers to show a current practising licence, or they could be voiding their insurance.