Looking for a historical data for groundwater quality parameters that recorded at high sampling frequency for a period of 3 to 6 months. Better if the provided data are measured near a landfill site.
Most regional councils in New Zealand should be able to provide high sampling frequency data. Some may have a site close to a landfill. You could visit www.enviroknowledge.co.nz site and find the regional councils' website links under NZ Environment followed by Functions and Powers and followed by Regional Councils. Once you are on one of the regional councils' website you could contact them with your query.
Although there are some websites or organization which are dealing with that but mainly depends which study area you need the water quality parameters. Like