An underwater sonar device, lost while scouring the southern Indian Ocean floor for missing flight MH370, has been recovered almost four kilometres below the sea surface.

The "towfish" was collected on April 18, nearly a month after it was lost, federal transport minister Darren Chester said on Wednesday.

"The equipment will be examined and tested, and then the vessel will return to the search area," Mr Chester said.

A remotely-operated underwater vehicle was used to locate and recover the missing equipment in water depths of 3700 metres.

The Australian Transport Safety Bureau also confirmed two objects that could be from the Malaysia Airlines flight, found in South Africa and east of Mauritius on Rodrigues Island, arrived in Australia last week.

The South African find has part of a Rolls Royce logo on it while the Rodrigues Island discovery could be the first piece of interior debris from the Boeing 777.

Authorities recently confirmed two pieces of debris found in Mozambique were "almost certainly" from MH370, given unique stencilling on both fragments.

The search has now covered more than 100,000sq km of seafloor, with only 20,000sq km left to be scoured.

The Australian, Malaysian and Chinese governments have agreed the search area will not be expanded if no new credible information is uncovered between now and when the search ends in mid 2016.