According to this article, NZ was offered British aid to build a small nuclear reactor in the mid-1940s for "medical, industrial and scientific purposes".
The real purpose behind this is not hard to fathom. After having been a junior partner in the Manhattan Project to build the first atomic bombs, Britain had been cut off from access to nuclear material and knowledge by the McMahon Act . As a result, an independent project was started to build a bomb, the first step in this being the construction of nuclear facilities at Windscale.
Canada was involved in this from an early stage, both in mining uranium and in building reactors, the plutonium from which was sold to both Britain and the US. The whole nuclear weapon program was very difficult for a small and relatively impoverished country like post-war Britain, and any assistance, especially in terms of space for facilities was obviously welcome.
Fortunately, as with later British requests for a test site, NZ declined this "opportunity".