Two shipwrecks discovered by accident during the search for missing Malaysia Airlines flight MH370 have been confirmed to be 19th-century coal-carrying merchant sailing vessels.

The vessels were detected in May and December of 2015 during a search of the Indian Ocean seabed for signs of the lost Boeing 777, which disappeared in March 2014 with 239 people on board.

The wrecks were found about 35 kilometres apart at a depth of almost four kilometres, more than 2,300km off the WA coast.

Experts from the WA Museum were asked by the Australian Transport Safety Bureau to examine sonar and video data of the two wrecks — one of which had a wooden hull while the other was iron.

The wooden ship's metal fastenings such as anchors and cleats remain on the ocean floor. ( Supplied: Australian Transport Safety Bureau )

An explosion or other catastrophic event

WA Museum curator of maritime archaeology Ross Anderson said analysis of the wreckage, along with historical records, helped to identify the types of vessels.

But he said research into lost ships was incomplete, so determining the exact identity of the vessels themselves was difficult.

"However, we can narrow the possibilities to some prime candidates based on available information from predominantly British shipping sources," Dr Anderson said.

"For the wooden ship, the brig W. Gordon and the barque Magdala are two possible candidates.

"For the iron ship the barques Kooringa [1894], Lake Ontario [1897] and West Ridge [1883] are possible, with the West Ridge best fitting the evidence."

The hull of the wooden ship has totally degraded during its time underwater, leaving only the vessel's cargo of coal behind, as well as the ship's metal fastenings, such as anchors and cleats.

It is believed the vessel would have been in the 225- to 800-tonne range.

Dr Anderson said it was likely the ship sank as a result of a catastrophic event such as an explosion — a common danger in 19th-century coal transport.

"Most of the material widely scattered on the seabed consists of the remains of the coal cargo that spilled out of the hull prior to it striking the seabed," he said.

'Mystery chest' turns out to be water tank

Early analysis of the site identified a large rectangular metal object about six metres long, which was initially referred to as a "mystery chest".

Dr Anderson said the object, which was the biggest feature found at the wreck site, has now been identified as a water tank.

What was earlier described as a "mystery chest" turned out to be an iron water tank. ( Supplied: Australian Transport Safety Bureau )

The wreck of the iron-hulled ship was more intact, and found lying upright on the sea floor.

Sonar and video images revealed it had at least two decks, and weighed between 1,000 and 1,500 tonnes.

Some of the ship's deck rails and portholes were visible in the images.

Dr Anderson said both vessels probably carried crews of between 15-30, and would likely have taken on additional passengers as well, making it difficult to guess how many people may have died when the ships sank.