There are 2 main types of visa for entry into New Zealand. A Working Holiday Visa and a Tourist Visa. In order to work legally in New Zealand you must have a valid New Zealand Visa.
If you are a resident in one of the countries listed below you are able to apply for a Working Holiday Visa in New Zealand:
Argentina, Belgium, Canada, Chile, Czech Republic, Denmark, Estonia, Finland, France, Germany, Hong Kong, Ireland, Italy, Japan, Korea, Malaya, Malta, Netherlands, Norway, Singapore, Sweden, Taiwan, Thailand, UK, USA and Uruguay.
The cost of a Working Holiday Visa varies depending where you are a resident. You can apply online or find any further information at: http://www.immigration.govt.nz/
For most visitors you are not required to apply for a visa in order to visit New Zealand for a period of up to 3 months (6 months for UK citizens) for leisure purposes.
To double check that you are a citizen of a visa-free country please check: http://www.immigration.govt.nz/migrant/stream/visit/
Please note that you will be required to show evidence of onward travel.