"Skinnies went to market with this product after getting a valid test result for only one person," Consumer NZ chief executive Sue Chetwin said.
"It then found the SPF was degrading and had to reformulate."
She says given the discrepancies, Consumer expected manufacturers to immediately review their testing programmes, rather than attack the messenger.
Chetwin says Consumer commissioned an accredited lab to conduct a 10-person test of the sunscreens according to the Australian and New Zealand voluntary standard.
"Two companies, Le Tan and We Are Feel Good Inc, provided lab reports showing their products had been tested on 10 subjects and met their label claims before being put on the market."
The Cancer Society this week questioned the testing methods used by Consumer NZ.
CEO Mike Kernaghan said Consumer decants the product in New Zealand before sending it for testing in Australia - whereas the society's own tests, where bottles are sent as is to the US, came back with a 60+ rating.
However, Chetwin responded by saying it was common for samples to be sent this way - and it was packed according to the lab's instructions.
"Preliminary results from three subjects showed it is likely to meet its SPF50+ label claims, but it hasn't provided a full 10-subject test."
University of Auckland Professor of Biostatistics Thomas Lumley says even in 10 person tests, there is still some variability.
He says there is potential for variation between batches and there are still questions over how big variations can be between labs.
Consumer NZ is asking the Government to make the sunscreen standard mandatory as part of the upcoming review of therapeutic products.
