JAKARTA - Indonesian navy divers on Thursday (Nov 1) recovered the black box of downed Lion Air flight JT610 which holds vital clues on what caused Indonesia's worst aviation disaster in two decades.

All 189 people on board the Boeing 737 Max 8 plane are feared dead after it crashed into Java Sea shortly after take-off on Monday morning.

"We followed the device, and narrowed the area (of search) and then we dug again the location where the sensitive (ping) sound was heard and finally found the blackbox," said diver Hendra Saputra, who found the black box.

He added the black box was found in good condition and at a depth of about 30m.

There are two so-called black boxes on each plane, and they are actually bright orange. One records conversations in the cockpit, while the other tracks crucial data like airspeed, altitude and fuel flow.

The one recovered on Thursday is the flight data recorder, Transportation Minister Budi Karya Sumadi said.

Retrieving the black box will be key to an ongoing investigation into the cause of Monday's crash, one of the worst in the country's aviation history.

The head of Indonesia's national search and rescue agency Basarnas meanwhile told reporters that apart from the black box, divers also found a large object measuring 1.5m by 0.5m that could be part of the fuselage.

The biggest question following the crash, the first for Boeing 737 Max 8, is why a plane that started flying just two months ago could have crashed.

The new aircraft type fitted with a new engine type had entered into commercial service in 2017 and the one that plunged into the sea was delivered to Lion Air only in August this year.

Before Monday's accident, the plane had experienced problems with the sensors used to calculate altitude and air speed on a flight from Bali to Jakarta on Sunday night. One passenger on board Lion Air flight JT43 took to social media to express her anger and disbelief that the plane was put into use again barely seven hours after her nightmarish experience.

Mr Budi, the transport minister, said his team has checked all Boeing 737 Max 8 aircraft in the fleet operated by Lion Air and found no technical problem.

"But, our evaluation will be forwarded to the KNKT and we will also discuss it with the Boeing team," he told a press conference, referring to Indonesia's National Transportation Safety Committee (KNKT). He added that a more detailed assessment of the aircraft will be conducted.

KNKT chief Soerjanto Tjahjono said on Thursday his team will meet representatives from the aircraft manufacturer Boeing and the US National Transportation Safety Board who have arrived in Jakarta to help out with the investigation, which KNKT says would take up to six months.

"The aircraft is a new type. So we need much explanation from the manufacturer," he told reporters in a Kompas TV broadcast.

The national search and rescue agency Basarnas operational director Bambang Suryo Aji on Monday ruled out the possibility that the plane exploded in mid-air, as the debris and human remains found showed no signs of being burnt.

The search for black boxes normally takes days and even weeks.

The black boxes of Air Asia QZ8501, which plunged into the Java Sea en route from Surabaya to Singapore on Dec 24, 2014 and killed 162 people on board, were found after 16 days of search.

Basarnas chief Muhammad Syaugi told reporters earlier that the search on Thursday morning will focus on sector four of the search area that is divided into 13 sectors.

"(Sector four) is the area where we detected what we suspect is a body part of the plane. And also where we picked up the 'ping'," he said, referring to the sound emitting from the black boxes and picked up by underwater "pinger locators".

The search area has been doubled in size to cover a 10-nautical-mile-radius area with the centre located at the point where the plane lost contact with air traffic controllers shortly after take-off on Monday morning.

Syaugi told Elshinta radio on Thursday morning that the search teams will have a better chance of recovering the black boxes on Thursday after they received the go-ahead from state oil company Pertamina to throw an anchor to stabilise the main search and rescue vessel.

"Yesterday afternoon, the currents were strong. We had to throw an anchor, but we only had clearance to do so last night from Pertamina. The sea floor there is lined with Pertamina pipes," head of the national search and rescue agency (Basarnas) Muhammad Syaugi told Elshinta radio on Thursday morning.

The Boeing 737 Max plane crashed into the Java Sea en route from Jakarta to Pangkal Pinang, Bangka Belitung Islands, on Monday. The wreckage is believed to be submerged at a depth of 30m to 35m underwater.

The plane is believed to have broken up into pieces upon severe impact with the sea surface after it dived at a rapid speed from a height of around 900m.

Data from Flightradar24, a flight tracking system, showed that the plane fell from 4,825 feet in less than 25 seconds, with the velocity reaching 480kmh.

"The assumption is that it was impossible (for the plane) to stay afloat, which would have happened had there been propulsion (into the water) because of engine pressure," Senior Basarnas official Didi Hamzar said as reported by kompas.com.

Officials have said the discovery of body parts suggested a high-impact crash in water off the coast of Indonesia's Java island.

Jakarta Police medical division chief Musyafak in an update on identification efforts on Wednesday said 56 body bags have been received so far, and 238 DNA samples taken.

There have been no burn wounds found on any of the remains in the 48 bags already examined, and eight more will follow on Thursday.

Psychologists are also on hand to counsel family and affected staff.

"Some family members have accepted (the situation). Others still have not come to terms with it," said Brigadier-General Musyafak.

"There is a 70 per cent likelihood that we will find the black boxes," said Soerjanto, whose committee is leading the investigation in a country plagued by aviation mishaps.

"The other 30 per cent will be our prayers," he was quoted as saying by Jakarta Post.

Although the currents in the waters off Tanjung Karawang, West Java - where the plane crashed with 189 people on board on Monday morning - were not heavy, the team of divers faced some difficulties during the operations, a KNKT official said.

"(Underwater) visibility is very restricted, which is why the process (of finding the black box) is taking quite some time," KNKT accident investigator Ony Suryo Wibowo said on Tuesday.

"The sea water was clear, but when we dove further towards the seabed, it was mud. We moved a little, the water quickly got murky," Mr Sudaryanto, one of divers, told Elshinta radio.

On Tuesday, the divers could work from early in the morning until late in the afternoon because the wind was too strong, Ony said.

Specialised equipment, such as robots, to detect the black box was deployed after nightfall and when the tide was high, he said, reported Jakarta Post.

"But the limited visibility also makes it difficult for the equipment to detect anything," Ony added.

West Java Police chief Inspector General Agung Budi Maryoto said the muddy waters made it hard for divers to see things clearly.

"The weather is clear and the waves are calm, however, the mud makes things a little bit difficult," Agung said as quoted by kompas.com.

While all 189 people on board are feared dead, at least one family received some closure after police confirmed the identity of a 24-year-old passenger from her remains.

Police Brigadier-General Hudi Suryanto said they managed to match her fingerprints to government records, identifying her as Indonesian civil servant Jannatun Cintya Dewi.