The Ministry of Health is preparing prosecutions against retailers who were recently shown on television selling cigarettes to people under the age of 18.
Some of the retailers were caught in a controlled purchase operation filmed by TVNZ's Sunday programme, while others were shown on Campbell Live on TV3.
Director General of Health Stephen McKernan says 20 retailers selling cigarettes to minors have been prosecuted so far this year, and prosecutions involving another 20 are underway.
Mr McKernan says the latest cases should serve as a warning to retailers.
He says the Health Ministry is stepping up controlled purchase operations, where underage volunteers try to buy tobacco.