Jonti Roos described the encounter on Australia's "A Current Affair". The airline said it wouldn't comment until its investigation is complete.
Roos said she and her friend were allowed to stay in the cockpit during the entire one-hour flight on Dec 14, 2011, from Phuket, Thailand, to Kuala Lumpur. She said the arrangement did not seem unusual to the plane's crew.
"Throughout the entire flight, they were talking to us and they were actually smoking throughout the flight,'' Roos said.
Roos did not immediately reply to a message sent to her via Facebook, asking for further comment on the incident.
The missing Malaysia Airlines Flight MH370 took off from Kuala Lumpur, on the western coast of Malaysia, early Saturday en route to Beijing.
It flew across Malaysia into the Gulf of Thailand at 35,000 feet (11,000 meters) and then disappeared from airport radar screens.
China, where two-thirds of the passengers are from, urged Malaysian authorities on Tuesday to "speed up the efforts" to find the plane. It has sent four ships to aid the search, with another four on the way.