The Fields Medal is often referred to as the 'Nobel Prize' of mathematics. It is only awarded to up to four mathematicians worldwide, and only every four years, for significant contributions to the study of mathematics by those under the age of 40.
The only New Zealander ever to receive one is Sir Vaughan Jones (b. 1952) who received his Fields Medal in 1990 for work done in the mathematical area called Topology. Topology is a branch of mathematics regarding shapes. It is often called 'rubber sheet geometry', and this has nothing to do with mathematician's propensity for bed-wetting. Rather, it's the idea that in Topology shapes are defined not by their geometric shapes, but by their complexity.