The move comes as part of Cadbury's international commitment to Fairtrade, which secures fair minimum prices for developing market commodity producers, and is used on a range of products such as coffee and chocolate.
Certification and labelling allows buyers to recognise and choose products that meet international Fairtrade standards.
In March, Cadbury in Britain announced plans for all Dairy Milk in Britain and Ireland to be sold under the Fairtrade logo by the end of its 2009 summer.
Fairtrade executive director Steve Knapp said Cadbury New Zealand's commitment to the movement was a landmark step.
"Cadbury's commitment to Fairtrade is life-changing news for cocoa farmers who will be able to sell more of their cocoa as Fairtrade, helping to improve living standards and create a better future for their families and communities," Mr Knapp said.
"This announcement sets a new standard for the mainstream chocolate industry in New Zealand and will open up new Fairtrade opportunities for cocoa farmers in our corner of the world."
The decision will transform Fairtrade chocolate from a niche product to a mainstream staple, progress which has been welcomed by humanitarian campaigners, including World Vision Australia chief executive Tim Costello.
"As consumers it is far more convenient for us to suspend belief and keep eating chocolate, but the uncomfortable truth is that most chocolate manufacturers still cannot guarantee that their chocolate is free from child labour," Mr Costello said.
"Fair cocoa pricing is the difference between children wielding a machete in the cocoa field or a pencil in the classroom."
The Stop the Traffik Australia Coalition said it was estimated tens of thousands of children were trapped working in slave labour conditions on cocoa plantations and farms in Ghana, the west-African nation where Cadbury will source its cocoa from ethically run plantations.
Cadbury New Zealand's managing director Matthew Oldham said the company was proud to be bringing the Fairtrade mark to its signature chocolate.
He said the change would come with no extra cost to buyers, and no change to the chocolate.
This month the New Zealand company bowed to public pressure to get rid of palm oil from its chocolate.
The firm started using palm oil recently as part of a cost-cutting exercise, replacing a portion of the cocoa butter ingredient, but returned to the original recipe following public outcry.
