The widow and sons of a passenger on missing Malaysia Airlines flight MH370 are suing the carrier in an Australian court, saying they have suffered "nervous shock", a document showed Wednesday.

Just days before the second anniversary of the plane's disappearance, Yen Li Chong along with sons Justin Jia Tian Tan and Javier Jia He Tan is claiming unspecified damages from the airline.

Her husband Chong Ling Tan was in the business class cabin of the jet when it disappeared on a Kuala Lumpur to Beijing flight on March 8, 2014.

The family are suing Malaysia Airlines under the 1999 Montreal Convention, which establishes airline liability for accidents.

"The defendant is liable for such nervous shock sustained by the first, second and third plaintiffs as a result of the death of the deceased," the writ filed in the Victorian Supreme Court states.

The document alleges the "aircraft would not have disappeared" if not for the carrier's negligence, which it said included failing to ensure the safety of the flight, that reasonable or adequate precautions were taken, and that the flight was "monitored and tracked at all times".

It added that the nervous shock suffered by the 49-year-old widow and sons, now aged 19 and 15, was a "direct and foreseeable consequence of the disappearance of the defendant's aircraft".

The writ, filed on Friday and obtained from the court by AFP on Wednesday, does not state the nationality of the plaintiffs, but the family live in the Australian state of Victoria.

It is not the first case involving MH370, which mysteriously vanished with 239 people onboard.

In October 2014, a Malaysian family sued the carrier for negligence over the plane's disappearance, in what was believed to be the first lawsuit filed over the disaster.

Malaysia in January last year declared all on board to be presumed dead, which the government said would allow relatives to seek compensation and otherwise move forward.

The jet is thought to have crashed into the remote southern Indian Ocean but an Australian-led effort scouring the area has so far been unable to locate any wreckage.

Only a two-metre-long (almost seven-foot) flaperon wing part that washed up on a beach on the Indian Ocean island of Reunion in July has been confirmed to have come from aircraft.