MELBOURNE, Australia — As Australia prepares a yearlong search for the jet carrying Malaysia Airlines Flight 370 in an area of ocean floor the size of Croatia or West Virginia, a government report underscores the lingering uncertainty about where the plane went down, noting that it might have fallen in an area of the southern Indian Ocean up to 19 times as large as the new search area.

Warren Truss, Australia’s deputy prime minister, announced plans on Thursday for an exhaustive search of the areas of seafloor with the highest probability of containing debris from the Boeing 777-200, which vanished on March 8 with 239 people aboard. That search is to cover an area of 23,000 square miles.

“The search area has at all times been based on the best information,” Mr. Truss said at a news conference in Canberra, Australia, heading off questions about why the search area keeps moving.