The aircraft are working with local vessels at the scene of the sinking, about 86km northeast of the Tongan capital Nuku'alofa, after a distress call was received at about 11pm (1100 GMT) on Wednesday, officials said.
The ferry sank fast "but we don't know why," Neville Blackmore, spokesman for the New Zealand Rescue Coordination Centre (RCCNZ) said in a statement early onThursday.
New Zealand is responsible for search and rescue operations in the Tongan region.
Search and rescue
According to the RCCNZ, the ferry was carrying 79 people, with 52 having been rescued so far.
The ferry had been travelling from Nuku'alofa to Ha'afeva, in the Nomuka Islands group.
Three ships are at the scene looking for survivors in the 270km sq search area, the RCCNZ said.
Weather conditions are to be moderate, with a two to three meter swell, forecast to ease, and a water temperature of around 25C.
PHOTO CAPTION
File photo of Princess Ashika, the ferry that sank 86 Kilometers (53.4 miles) northeast of Tongan capital Nuku'alofa, taken on July 1, 2009.
Al-Jazeera