Seven people were rescued last Sunday and Kiribati authorities have now confirmed the boat was carrying more than 80 people, including many school children.
It is thought to be the worst maritime tragedy in the island nation's recent history.
There are reports that the ferry which had just been repaired, was overloaded and did not have an emergency beacon.
The 17-metre-long wooden catamaran, the MV Butiraoi, left Nonouti Island on January 18 for the main island of Tarawa on a two-day journey. Source: Supplied
Clinging onto hope against the odds is Wanita Limpus, whose nephew has been missing at sea for 13 days.
"My nephew Philip, he's fifteen, turning sixteen this year, he's a lovely boy and got a good character as well,” said Ms Limpus who lives in Queensland and is a founder of the Kiribati Australia Association.
"I’m really sad and the emotions are really high, the feeling in the community is grief, anger.”
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High school student Philip was travelling in preparation for the start of term.
Kiribati authorities on Wednesday confirmed there were 88 people aboard the 17-metre wooden catamaran MV Butiraoi.
Four planes from Australia, New Zealand and the US and six boats are searching for more than eighty passengers, including twenty-three school children.
The ferry was on a two-day voyage from Nonouti Island to the capital Tarawa - where rescuers initially focused search efforts - but two weeks on the search area now covers thousands of kilometres of sea west of Kiribati.
Seven people were rescued on Sunday after being spotted by a New Zealand defence force plane. Source: Maritime New Zealand
Seven people in a dinghy were rescued on the weekend only after Kiribati authorities raised the alarm on Friday, more than a week after the ferry sunk.
Rescue authorities hope a life raft capable of carrying 25 people may still be found, but the survivors said a second dinghy capsized after the ferry sunk.
The tiny Pacific nation of Kiribati has a population of just over 110,000 people, a tragedy such as this touches almost everyone in the country, and the government has declared a week of prayer.
Wanita Limpus with a photo of nephew Philip who is missing after the sinking of a inter-island ferry. Source: Stefan Armbruster SBS
"At the moment it is hard to grieve because he [Philip] might still be alive, and others might still be alive," said Ms Limpus.
"We just wait and see, but I'm not holding out much hope.
"If there is anything positive come out of this then it's lessons learnt, but if nothing learnt then these accidents are going to keep happening."
A full investigation into the sinking has been promised by the Kiribati government.
