Locking up Aboriginal children costs Australia almost a quarter of a billion dollars a year, according to new figures compiled by Save the Children and released on the eve of Close the Gap Day.

On an average day last June, there were 480 Indigenous young people behind bars, with each young person costing authorities $1355 a day.

Government data shows that Indigenous young people are 26 times more likely to be incarcerated than non-Indigenous young people. Most of those Indigenous young people in detention are yet to be sentenced.

And according to the most recent snapshot, Indigenous young people made up 54 per cent of young people in youth justice detention last June, despite comprising about 5 per cent of the population.