The 17-metre (56-foot) catamaran was reported missing on Jan 20, two days after it departed Nonouti Island on a 250 km (155 miles) trip to Betio in Kiribati.
Seven people rescued from a drifting dinghy on the weekend said the ferry broke up soon after setting out on Jan 18 and that they had seen other passengers scramble aboard a life raft which was now the focus of the aerial search.
The New Zealand Defence Force had previously reported eight people had been rescued but had revised that number. Reports on the total number of passengers had varied in recent days between 35 and 100.
The Rescue Coordination Centre of New Zealand (RCCNZ), which is supporting the search, said in a statement on Wednesday that Kiribati authorities had confirmed about 80 passengers and five crew were aboard the ferry and seven of them had been rescued.
Two Australian aircraft and one from the US coastguard were combing around 92,000 square km (35,500 square miles) of the ocean for the missing liferaft, which was designed to hold about 25 people but capable of squeezing on more.
“It is still a rescue mission, not a recovery,” Vince Cholewa, a spokesman for the RCCNZ told Reuters by phone.
“Obviously the longer it goes the smaller those chances become but there’s still hope for finding survivors.”
A New Zealand plane spotted a dinghy on Sunday with seven people, including a 14-year-old girl, who had been adrift for days without water.
The seven were rescued by a nearby fishing vessel and were now making their way back towards land on a Kiribati patrol ship that was expected to arrive on Friday.
The survivors had been assessed by medics on board the patrol boat and were not in need of urgent hospital treatment, the RCCNZ spokesman said.