Australian authorities say a deep-water search for the missing Malaysia Airlines flight MH370 is on track to begin in September.

The plane, carrying 239 people on board, disappeared without trace over the southern Indian Ocean on March 8.

Since May two ships, Zhu Kezhen and Fugro Equator, have been mapping the sea floor where the plane is thought to have crashed, in preparation for the deep-water search.

Experts have narrowed their search to an area along the seventh arc - a thin but long line that includes all the possible points where the last known communication between the aircraft and a communications satellite could have taken place.

Australia's Joint Agency Coordination Centre (JACC) says it is necessary to map the sea floor in the remote region, which until now has been poorly charted, before the deep-sea operation can commence.

"The aim is to identify significant features on the sea floor, which may present a hazard for the deep water vehicles that will be used for the search," the JACC said in a statement.

It says the Zhu Kezhen has surveyed over 25,000 square kilometres while the Fugro Equator has surveyed more than 43,000 square kilometres.

A third vessel, Malaysia's KD Mutiara, will join the mapping operation in August.

"It is expected that the bathymetric survey work will be completed by September," the JACC said.

"The deep-water search is expected to commence in September."

The Australian Government has set aside $60 million to pay a private contractor to carry out the deep-sea operation.