Photo: © 2009 AAP/TRACEY NEARMY

The search is underway for two landing gear doors, which measure 30cm by 60cm and are believed to have fallen off while the plane was flying over Victoria's Gippsland region.

The Defence Force has no explanation as to why the doors came off, and issued a statement saying the incident is "unusual" and under investigation.

"An initial ground search has been conducted based on best known coordinates, and an aerial search will be conducted early next week," a spokeswoman told the Herald Sun.

"Due to the area involved and size of the components, you would appreciate that it would be very difficult to locate them by either air or ground search."