About 80 passengers and a crew of five were on board the ferry Butiraoi when it sank on January 18, Kiribati authorities have confirmed.
Seven survivors were rescued by the New Zealand Defence Force on January 28.
The search continues for a life raft that was launched from the sinking ferry. The life raft is designed for 25 people but more can be squeezed in uncomfortably, according to Maritime New Zealand.
Rescue Coordination Centre New Zealand (RCCNZ) is supporting RCC Nadi in Fiji, which is leading the search, and the Kiribati authorities.
Neville Blakemore, RCCNZ Watch Leader, said an RNZAF P-3 Orion had been searching for four days. In addition, three further aircraft, from the RAAF, the Australian Maritime Safety Authority (AMSA) and the United States Coast Guard (USCG), joined the search more recently and are continuing on January 31.
The ferry left Nonouti Island on January 18 for a two-day, 250-kilometer trip to Betio. The wooden catamaran failed to arrive on January 20.
Many of the missing passengers would have been kids on their way to the high school in Tarawa, Ieremia Tabai, MP, was cited by Radio New Zealand as saying.