Ultra Violette’s Lean Screen SPF 50+ Mattifying Zinc Skinscreen, a facial product that Rach says she used exclusively, was the “most significant failure” identified. It returned a result of SPF 4, something that shocked Choice so much it commissioned a second test that produced a similar reading.

Other products that did not meet their SPF claims included those from Neutrogena, Banana Boat, Bondi Sands and the Cancer Council - but they all rejected Choice’s findings and said their own independent testing showed their sunscreens worked as advertised.

An investigation by the Australian Broadcasting Corporation found that a single US-based laboratory had certified at least half of the products that had failed Choice’s testing, and that this facility routinely recorded high test results.

Everyone’s skin responds differently to the product, she adds, and it’s one that is almost always being stress-tested - by sweat, water, or makeup.

It is very difficult to rate effectively for the same reasons. Historically, it has been done by spreading the sunscreen on 10 people at the same thickness, then timing how long it takes for their skin to start burning both with and without the product applied.

While there are clear guidelines as to what you are looking for, Dr Wong says there is still a lot of variability. That is down to skin texture or tone, or even the colour of the walls, and “different labs get different results”.

But she says results are also quite easy to fake, pointing to a 2019 probe by US authorities into a sunscreen testing laboratory which resulted in the owner being jailed for fraud.

Many sunscreen brands from all over the world use the same manufacturers and testing labs - and so this issue is unlikely to be isolated to Australia, she adds.