Contact informationConcerned family members and friends should contact Malaysia Airlines on +603 7884 1234 (international call) for further information.
The Department of Foreign Affairs and Trade (DFAT) said it was in contact with the families of the six Australians, and that officials are in "constant and urgent contact" with Malaysia Airlines.
No distress signals were received from the aircraft before its disappearance, and DFAT said it "fears the worst" for those onboard.
The ABC understands one of the Queensland couples is from Brisbane, while the other is from Springfield Lakes.
Flight MH370 lost contact with air traffic controllers at 2:40am local time (5:40am AEDT) on Saturday, just over two hours into what should have been a six-hour journey.
The airline said the last point of contact with the plane was 120 nautical miles off Kota Bharu, over the South China Sea.
A full-scale international search-and-rescue effort is underway, with ships and aircraft from Malaysia, Vietnam, China and the Philippines taking part. Singapore is helping to coordinate the search.
Malaysia's acting transport minister, Hishammuddin Hussein, has rejected reports that the plane's wreckage had been found off the Vietnamese coast.
He said the Malaysian navy and air force, along with the Chinese navy, are searching for the plane in the region where it lost contact.
"We are deploying all the ships in the vicinity to the areas concerned. Our coast guard is being deployed. Our 725 helicopters have also been deployed," he told a press conference.
He also said reports that the plane had landed in southern China were not true.
The plane, a Boeing 777-200, left Kuala Lumpur at 12:41am on Saturday and had been due to arrive in Beijing at 6:30am local time.
Malaysia Airlines said the plane was carrying 227 passengers, including two infants, plus 12 crew members.
The airline said the flight was piloted by captain Zaharie Ahmad Shah, 53, who had flown with Malaysia Airlines since 1981. He had a total of 18,365 flying hours.
The first officer was 27-year-old Fariq Ab Hamid, who joined the airline in 2007 and had flown a total of 2,763 hours.
Malaysia Airlines contacting passengers' next of kin
The airline said it was contacting the next-of-kin of all passengers and crew, which includes people of 14 different nationalities.
There were 154 passengers from China and Taiwan onboard, while there were 38 from Malaysia and seven from Indonesia.
Two New Zealanders were also on the flight.
In a statement on the airline's website, group chief executive officer Ahmad Jauhari Yahya said the airline was working with authorities to locate the aircraft.
"[The] focus of the airline is to work with the emergency responders and authorities and mobilise its full support," the statement said.
"Our thoughts and prayers are with all affected passengers and crew and their family members."
The airline said it would provide regular updates on its website.
Earlier reports from China's Xinhua news agency said the plane was lost in airspace controlled by Vietnam.
Xinhua said the aircraft did not enter airspace controlled by China and did not make contact with Chinese controllers.
Distressed family members of those onboard the flight began gathering at Beijing airport early this morning.
I got here at 7:00am. At first I thought the plane was just delayed as normal, so I came a bit later. I've just been waiting and waiting.
Chang Ken Fei, who had friends on the flight
Chang Ken Fei, a Malaysian waiting at the airport for friends to arrive, said: "I got here at 7:00am. At first I thought the plane was just delayed as normal, so I came a bit later. I've just been waiting and waiting".
"I asked them what was going on but they just tell us: 'We don't know'."
If the plane is found to have crashed, the loss would mark the second fatal accident involving a Boeing 777 in less than a year, after an unblemished safety record since the jet entered service in 1995.
Last year, an Asiana Airlines Boeing 777 crash landed in San Francisco, killing three passengers.
Boeing said it was aware of reports that the Malaysia Airlines plane was missing and was monitoring the situation, but had no further comment.
Last October, another Malaysia Airlines plane, a smaller Twin Otter aircraft, crashed upon landing in Malaysia's Sabah state on Borneo island, killing a co-pilot and a passenger.
A Malaysia Airlines jet crashed in 1977 in southern Malaysia, killing all 93 passengers and seven crew.