The search for the missing Malaysia Airlines plane MH370 has found a previously unknown shipwreck at the bottom of the Indian Ocean.

An underwater vehicle being towed by one of the search boats detected several unusual objects more than four kilometres below the surface and 1000 kilometres off the coast of Western Australia.

"We were cautious about this," said the director of the MH370 search Peter Foley, from the Australian Transport Safety Bureau.

"There were characteristics of the contact that made it unlikely to be MH370, but there were also aspects that generated interest, multiple small bright reflections in a relatively small area of otherwise featureless seabed," he said.

Another search boat, owned by the Dutch company Fugro hired to conduct the search, was diverted to investigate. Images taken by an automated underwater vehicle and analysed this week reveal several man made objects including an anchor, and a box-shaped object about six metres long. Black rocks scattered across the seabed are thought to be coal.

The ATSB said the wreck is previously uncharted and the images will be given to expert marine archaeologists in the hopes they could identify it.

"It's a fascinating find but it's not what we're looking for," said Mr Foley.

Flight MH370 disappeared on March 8 last year with 239 people on board while travelling from Kuala Lumpur to Beijing. Authorities believe the plane crashed into the Indian Ocean off the coast of Western Australia.

Since then search boats have scoured about 75 per cent of the initial 60,000 square kilometre search area. In April, Australia, Malaysia and China agreed to doubled the search area to 120,000 square kilometres if the plane is not found in the current high priority area.

"We're disappointed that it wasn't the aircraft, but we were always realistic about the likelihood," said Mr Foley.

"We're not pausing in the search for MH370, in fact, the vessels have already moved on to continue the mission."

Mr Foley said this event demonstrated that the systems, people and the equipment involved in the search were working well.

"It's shown that if there's a debris field in the search area, we'll find it."