Fifteen survivors of an overloaded migrant boat that sank off eastern Java on Saturday were found in two separate islands on Monday.

Thirteen of the survivors were picked up from the Nusa Barung island, about 200 kilometres from the site of the shipwreck, while two crew of the ill-fated boat were found on the East Java mainland at Sindang Biru, according to Indonesian rescuers. The 15 were transferred to the town of Puger for treatment at a local hospital.

Meanwhile, Home Affairs Minister Jason Clare said a plane and a ship will join boats and helicopters of Indonesia's National Search and Rescue Agency in the search for others missing at sea. The search area has been expanded to cover the coast of Bali and islands further east, such as Flores.

Australian police will also be sent to East Java to find out who organised the attempt to smuggle the asylum seekers, who are mostly Afghans and Iranians.

The boat was heading to Christmas Island when rough waves caused it to list and start sinking at 6 a.m. on Saturday. The captain and crew abandoned the boat and the passengers, according to survivors.

Fishermen rescued 33 survivors, who were clinging to a wreckage for about eight hours. The survivors were taken to the town of Blitar.

Fifteen bodies were also found in the water on Saturday.

The asylum seekers have told Indonesia authorities that they have paid Indonesian people smugglers $6,000 each to be transported to Christmas Island.

Fifteen survivors were found in Nusa Barung island off east Java.