The Cliflo API from New Zealand's National Institute of Water and Atmospheric Research (NIWA) gives access to data from 6,500 climate stations maintained by the government of New Zealand. The API webpage notes that access is by subscription, though the cost is free, suggesting that only registration and agreement with the terms and conditions are required. Access to data gathered by Pacific Island stations is restricted by agreement with Pacific Island countries. However it is possible to obtain access by permission.
Why bother paying attention to climate in New Zealand (unless you live there)? As NIWA's biological oceanographer Dr. Philip Boyd explains in an interesting video that phytoplankton play an important role in regulating the climate "by sheer weight of their numbers." Because southern oceans are Earth's warmest, they contain more phytoplankton, hence the region is important for research.
Little technical information about the API is available. However, there is an extensive guide on how to make queries, called Cliflo for Dummies. The earliest observations in the database go back to 1850.
Established in 1992, NIWA is a Crown Research Institute (CRI), owned by the New Zealand government. I carries out research and activities in a number of areas from aquaculture to climate, from energy solutions to Te Kūwaha, NIWA's Māori Environmental Research Group, which assists Māori communities by providing science to aid in managing natural resources.
The scientists in the organization were recently featured in a 13-part series on Maori TV.