Can somebody explain me if there is any known reason for the high prevalence of the skin melanoma in Queensland and Newzeland?
One of the causes of melanoma is exposure to UV radiation , and Australia is a sunny place!
This reference is also helpful:
"Australia has one of the highest skin cancer incidence and mortality rates in the world. The reason for these high rates is due in part to the high ambient solar UV levels, combined with a predominately susceptible fair skinned population. With a population of just over 20 million, over 1700 Australians die from skin cancer each year. At least two in three Australians will be diagnosed with skin cancer before the age of 70 and over 400 000 Australians are treated for skin cancer each year." 
Sinclair, C.; Foley, P. "Skin cancer prevention in Australia". British Journal of Dermatology. 161: 116–123 (2009)