A power struggle has emerged in the Malaysian-led investigation into the disappearance of missing Malaysia Airlines flight MH370 over the Indian Ocean four years ago.

Key points: Four civilian air crash investigators sidelined over reported budget constraints

Four civilian air crash investigators sidelined over reported budget constraints Malaysian military wants to replace them with fighter and helicopter pilots

Malaysian military wants to replace them with fighter and helicopter pilots Figures linked to the investigation are concerned it has tarnished the search's independence

Four civilian air crash investigators, including the lead authority on analysing black box flight data, have been sidelined over reported budget constraints.

The Malaysian military wants to replace them with seven Royal Malaysian Air Force fighter and helicopter pilots with far less crash investigation experience.

The lead aviator, Colonel Lau Ing Hiong, confirmed his secondment to the search team

"Yeah, it's correct," he said.

The colonel said he saw his role on the team as being there in anticipation of the black boxes being found, but denied there would be a military operation to secure them.

Figures linked to the investigation have told the ABC on condition of anonymity they are concerned the recent move has tarnished the search's independence.

A relative of a passenger onboard Malaysia Airlines flight MH370 cries as she talks on her mobile phone. ( Reuters: Kim Kyung-Hoon )

One said unauthorised people like Air Force personnel on board the search vessel could raise questions about the chain of evidence.

That is due to a perceived conflict of interest for military personnel between the civilian chief of the search and their military commander.

But insiders say the search has already been muddied merely because of the presence of military personnel on the team.

Search ship has been centre of conspiracy theories

The director-general of Malaysia's civil aviation authority declined an extended interview, but said: "These investigations need to be done by independent bodies."

The ABC has approached the Malaysian Prime Minister for clarification about the role of the Air Force.

It is not unprecedented for Malaysian Air Force personnel to be involved in air crash investigations — they were in the case of MH17, which was shot down over Ukraine.

The private sector vessel currently searching for the plane's wreckage, the Seabed Constructor, docked at Fremantle to refuel on Thursday.

Malaysian Transport Minister Liow Tiong Lai said so far the search had gone smoothly in the ship's 90-day race against the clock to find the wreckage.

"[In] one day they can search more than 1,000 square kilometres. We will continue to keep the public informed," he said.

The ship has been the subject of conspiracy theories after it turned off its transponder for 80 hours, fuelling internet speculation.

Malaysian authorities know why the ship was not reporting its location, but did not disclose the information in their weekly update on the search.

Until now, the MH370 search has been conducted by eight civilian aviation experts including Malaysians as well as foreign nationals, one of them from the Australian Transport Safety Bureau.