A sea of red crabs has started taking over Christmas Island as the annual breeding migration gets underway.

Rain has triggered the start of the season, and over the coming weeks millions of red crabs will move from the island's forest to the coast, to mate and spawn.

Christmas Island Tourism Association marketing manager Linda Cash said it was an extraordinary sight.

"It's a sea of red," she told ABC North West.

"I have seen it for 10 years and every time I see it I am just totally amazed.

"The sound, you can imagine millions and millions of crabs sort of crawling across the island, so the sound is quite extraordinary."

She said roads would be closed if the crabs started moving onto them.

"There are crab barriers too, they are just like plastic barriers that parks put along the road and the crabs can't actually climb over it and it forces them to go into the crab tunnels," she said.

"Christmas Island has a network of crab tunnels that allows the crabs to safely cross the roads.

"There are even crab bridges.

"Crabs will do whatever they need to do to get to their destination, so they will use the tunnels or the bridges if they are blocked by the barriers."

Ms Cash said the spectacle attracted people from all over the world.

"People plan for years to come and see this," she said.