It must be remembered that the period of our instrumental weather data is short when compared to say the ~15,000 years since the last ice age.
We must also remember that cold records are set in the face of the ever expanding urban heat island (UHI). So cold records are more remarkable than warm records that are assisted higher by the ever increasing UHI.
In this case the record could even more remarkable because the previous record in 1918 was set nearer the CBD in the Botanic Gardens, surrounded by suburbs – whereas since 1954 the instruments have been at the airport ~10kms away on the NW city fringe. UHI will be steadily creeping into the airport data too as suburbs grow and infill.
Thanks to Kiwis for tip off re NASA image.