The group was in a wooden speedboat in the waters around Sarang Alu and Banyak islands in Aceh Singkil district. They were part of a group of 12 Australian nationals and 5 Indonesians who were traveling to Pinang Island in two boats.
The area is known as a tourist destination with white sand beaches and good waves for surfing.
The boats left North Nias port in Nias island, which located around 150 kilometer (93 miles) from Indonesia’s Sumatra island, on Sunday afternoon and experienced bad weather with very heavy rain during the trip.
Ten of them decided to stay and shelter on Sarang Alu island, while the others continued the trip, said a written statement from Nias Search and Rescue Agency on Monday.
The resort on Pinang Island later reported to the agency that the boat with 10 passengers had safely arrived, but the boat that had left earlier had not been seen.
Rescuers were sending at least two rescue boats and a medical boat to the search area.
Indonesia is an archipelago with more than 17,000 islands, and ferries and boats are a common form of transportation. With lax safety standards and problems with overcrowding, accidents occur frequently.
In July, an overloaded passenger boat capsized off Indonesia’s Sulawesi island, killing 15 people.
In 2018, an overcrowded ferry with about 200 people on board sank in a lake in North Sumatra province, killing 167 people. In one of the country’s worst recorded disasters, an overcrowded passenger ship sank in February 1999 with 332 people aboard. Only 20 people survived.