Green’s Dictionary of Slang

(N.Z.) a school cadet uniform.

1941		cited in DNZE (1998) 692/2: St Patrick’s College, Silverstream. (Ed.) Sandpaper suit, a freq. term for the cadet uniform.
1983		Eve. Post (Wellington) 29 July 12: Whatever happened to those ‘sandpaper suits?’–the khaki shorts and battledress tunics worn by thousands of boys during the heyday of the school cadets. Most of the uniforms were manufactured during the Second World War and proved so durable that they were recycled... ‘They were issued to a cadet, returned at the end of the year, drycleaned and given to another cadet,’ said Major Don Stewart. He understood the uniforms went out of use some time in the 1960s [DNZE].
1995		(ref. to 1939) G. Slatter One More River 48: On 3 September 1939 [...] my khaki uniform was hanging [...] in the back bedroom, not the uniform of a real soldier but the ‘sandpaper suit’ of a secondary school cadet [DNZE].