The licencing of patents relating to radio manufacture prior to 1934 was not particularly well managed in Australia and New Zealand - and so in early 1934 the Australian Radio Technical Services & Patents Co. Ltd (ARTS&P) was formed by the patent holders, issuing a single manufacturing licence and allowing applicants on both sides of 'the ditch' to use any patents. ARTS&P also provided technical assistance to manufacturers so they could make the best use of the patented technology they were paying for.
The first year (1934) labels in NZ were printed on white paper and stuck on (while Australian 1934 labels were plastic and riveted to the chassis).
From 1935 until 1946 the labels were all pale blue, although the year can generally be approximated by the letter prefix on the labels serial number (which did not bear any resemblence to the radio serial number). From 1946 the licence labels changed color a few times, and this allows modern collectors and enthusiasts to at least ball-park age their sets if no other identifiers are available. The royalties paid to ARTS&P were based on the number of electron streams in the set, and this is represented by a number in the top left of the label.
While this system was used in Australia and New Zealand, there were some subtle differences between the two countries - most notable the wording, which either refers to the Commonwealth of Australia or the Dominion of New Zealand
The final label issued was blue and smaller than the previous labels (most likely because sets were becoming smaller, and real estate on a chassis was getting harder to find), but the use of this licensing system died out in the 60's and its common to find no trace of a label ever having been fitted to radios from the mid-50's on.
1934 White newsprint label, serial number has no prefix (white plastic label, 'A' prefix in Australia)
1935-1936 Serial number prefix 'B', pale blue label until 1946
1936 Serial number prefix 'C'
1937 Serial number prefix 'D'
1938 Serial number prefix 'E'
1939-1940 Serial number prefix 'F'
1940-1941 Serial number prefix 'G'
1942-1946 Serial number prefix 'H'
1946-1952 Dark green with red letters, serial number prefixed by the letter 'I'.
1952-1955 Orange with dark green letters, serial number prefixed by the letter 'I'
1955-1960s Small pale blue, with dark blue letters, no prefix to the serial number.
Information was gathered from various sources to produce this list:
More Golden Age of Radio - John W. Stokes