It is a myth that clouds protect fro UV rays. WHO says generally some clouds can reduce the intensity but other clouds may even enhance it.
Additional protection like tree shadow, sun milk, sun umbrella is given advice. The best will be: Don’t go out between 12h and 16h. Especially not when living in N.Z. Remember the ozone hole.)
UV levels are highest under cloudless skies, and cloud cover generally reduces a person's exposure. However, light or thin clouds have little effect and may even enhance UV levels because of scattering. Don't be fooled by an overcast day or a cool breeze! Even a long stay in open shade, for example between buildings, may give a sensitive person a sunburn on a day with high UV levels.