Four out of 10 most popular children's cereals are more than 30 per cent sugar, a new analysis shows.

The Obesity Policy Coalition analysed the sugar and salt content of 20 cereal brands and is urging manufacturers to adopt the voluntary health star rating system, which gives consumers at-a-glance information about a product's nutrition.

"Some of these products are very high in sugar and in salt and it is important people are given the tools to understand the overall healthiness of these products, particularly cereals," the coalition's executive manager Jane Martin said

Nutri-Grain, the second-highest selling cereal in Australia last year, had among the highest sugar content of the 10 children's cereals analysed.