Shampoos that promise to slow down colour fade have been kicked to the kerb in a new test looking at the best products to use to give a longer lasting hair colour.

And the most interesting result was that out of the 23 shampoos tested, none performed better than using just water to retain colour.

Consumer NZ carried out the test to see which shampoo for coloured hair gave a longer lasting colour. After 12 washes, none in the group performed better than an ordinary "all-round shampoo'' or even a shampoo designed for babies.

Consumer NZ chief executive Sue Chetwin said they found that "plain old water'' turned out to be the best solution.

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"Despite the marketing hype, we found washing your hair with water is the best option for freshly dyed hair,'' she said.

Each year, Kiwis spend more than $3 million on specialty shampoos for coloured hair.

The test found that even the expensive brands did not perform better than lesser-priced products and Ms Chetwin advised those hunting for colour-fighting shampoos to buy on price.

Brands tested included well-known shampoos made by Tresemme, Herbal Essences, Pantene, Garnier Fructis, Sunsilk and Schwarzkopf.

Other more expensive brands included Wella, Paul Mitchell, KMS California and SP System Professional, which was the most expensive at $14.40 per 100ml.