Many South Island melanoma patients have a different gene mutation than North Island patients and may end up being treated by different drugs.
A large study of patients led by the University of Otago has found similar BRAF gene mutation rates in both the North and South Island, but stark differences in the NRAS gene.
Mutations to the NRAS were found in 38 percent of South Island melanomas, but only in 21 percent of North Island ones.
Study co-leader Professor Mike Eccles of the Dunedin School of Medicine says the South Island rate sticks out "like a sore thumb", and is out of line with results in other countries as well.
Researchers don't think ethnic differences explain the mutation rates of the NRAS gene but sunburn or strong exposure to UV-radiation, especially during spring, may.
The most common mutation is to the BRAF gene, with one-third of the melanomas in the study showing changes to this gene, which may be targeted by the drug Zelboraf.
The researchers found NRAS mutations are linked to nodular and thus more deadly melanomas.
A preliminary 2015 US study found that in a small number of patients, melanomas with NRAS mutations had higher response rates to immunotherapy treatments such as the drugs Keytruda and Opdivo, Professor Eccles says.
"Our findings suggest that South Island melanoma patients could potentially benefit more often from the use of such therapies, should the US findings be confirmed by further research," he says.
The study, which is the first comprehensive genetic analysis of melanoma in New Zealand, sought to analyse mutation frequencies of 20 recurrently mutated genes in samples from 529 patients with metastatic melanoma.