The tiny Indian Ocean island of Mayotte will be the focus of attention across France when the French dependency holds a referendum Sunday on whether to become a French overseas department.

The growing number of illegal immigrants from neighbouring impoverished islands who arrive on this scenic island has however worried French authorities. According to some estimates, nearly one-third of the island’s 200,000 inhabitants could be illegal immigrants.

In 2008, about 16,500 people were turned back at the border, which is half the total number of expulsions registered by the French Immigration Ministry.

Dozens arrive every day and night from Anjouan, the Comoran island closest to Mayotte. Many of them risk their lives in small, unstable boats called "kwasa kwasa," sailing in dangerous, shark-infested waters. The tiny French enclave of Mayotte is rich, a virtual El Dorado by the standards of the Comoros archipelago.

Mayotte’s detention centre on the island of Pamandzi, is full to capacity. A new building is being constructed, which will be finished within two years. "People do not stay here for long - 24 hours, maximum," a police officer at the detention centre told FRANCE 24’s Lucas Menget.

On the island, where the laws are not the same as those in mainland France, migrants can be expelled within 12 hours and procedures cannot be appealed or suspended.

On a boat returning to Anjouan, a young man just released from the centre is nonplussed by his expulsion. "As soon as I arrive over there,” he said referring to his native Anjouan, “I return here."

And he’s not the only one. “In a week, we can see the same person returning two or three times," said a police officer at the detention centre.

For centuries, these islanders have traversed these waters, visiting neighbouring islands. But since 1995, residents of the islands around Mayotte cannot visit the French dependency freely. France has put in place fast patrol boats, radars and thermal night vision goggles to try to stem the flow of illegal immigrants.