Australian police seized 500,000 images and 15,000 videos showing the abuse of children as young as 12 months old.

The arrests were made in a series of raids around the country, following a one-year investigation into an online image sharing network.

"The videos seized as part of this operation have to be among the worst we have seen," said Australian Federal Police officer Andrew Colvin.

"Some depicted sexual abuse of children lasting more than two hours."