AirAsia plane
AirAsia planeReuters

Indonesia's top rescue official said on Monday morning that authorities believe the missing AirAsia jet is likely at the bottom of the sea, based on radar data from the plane's last contact.

"(Because) the coordinate that was given to us and the evolution from the calculation point of the flight track is at sea, our early conjecture is that the plane is in the bottom of the sea," said Marsdya Tni Hendry Bambang, head of Indonesia's national search and rescue agency, according to CNN.

Indonesia resumed at dawn Monday the search for the plane that went missing in the Java Sea with 162 people on board a day earlier, a search and rescue official told AFP.

Australia joined the Indonesia-led search for the plane, the military said.

A Royal Australian Air Force (RAAF) AP-3C Orion took off from the northern city of Darwin early Monday to join the operation, which is centered on the Java Sea, the Australian Defense Force said, according to AFP.

Chief of defense Air Chief Marshal Mark Binskin said the Orion would help in the search for the AirAsia flight, which went missing in bad weather en route to Singapore.

"The RAAF AP-3C Orion aircraft has a well proven capability in search and rescue and carries maritime search radar coupled with infra-red and electro-optical sensors to support the visual observation capabilities provided by its highly trained crew," he was quoted as having said.

Australian Prime Minister Tony Abbott has pledged to help Indonesia in the search for the missing flight, calling President Joko Widodo on Sunday to offer assistance.

Abbott told Widodo Australia would do "whatever we humanly could to assist," his office said in a statement.

Australia is already leading the search for another missing aircraft, Malaysia Airlines Flight MH370, which disappeared in March with 239 people on board and is believed to have crashed in the remote Indian Ocean far off Australia's west coast.

No sign of that plane has so far been found and a range of theories as its fate have emerged – from hijacking to crashing to being diverted for use in a terror attack, possibly against Israel. Later theories have become even wilder, ranging from an onboard fire to a suicide mission

Controversy reigns over every detail of the flight, including the co-pilot's last words - "All right, good night" - and the fact that two Iranian nationals with stolen passports were on board.

The latest publicized theory in June predicted that the 239 passengers and crew died from hypoxia, or oxygen deprivation, building on earlier analysis pointing to fuel starvation. A Helios Airways flight in 2005 crashed under similar circumstances.