A world leading skin cancer expert says SPF may not be the best way to rate sunblock.
Consumer New Zealand has tested 10 sunscreens and found six don't match the claims on the bottle.
The Cancer Society is also concerned some people are using an SPF six sunscreen unwittingly, because the bottle falsely claims it offers SPF 30 protection.
Skin cancer expert Dr Sharad Paul told Newstalk ZB's Mike Hosking it's not acceptable that one says it has an SPF of 30, but when tested only has six.
"Broadly speaking, you shouldn't have something which says it is SPF 30, and when tested it is only six. The NZ environment is high UV, which means you need a sunscreen with good protection.
Sunscreen is an imperfect science and people can get a bit too technical about it. A better way to rate protection could be labelling it as low, medium or high. It is frustrating sunscreen has no minimum standard set by the Government."
One type of the "Coola" brand sunscreen was found to have a single digit SPF, and Dr Paul said there are many sunscreens out in the market which don't live up to their claims.
"Education is really important. We should test score better so people know where they stand as it is currently to confusing for the consumers who don't know if they are buying a product which works."
New Zealand's melanoma incidence rate is the world's highest, with 4000 people diagnosed with melanoma every year.