HONG KONG — While BP tries various short-term efforts to plug a leaking oil well in the Gulf of Mexico, the company is preparing to drill a relief well as a backup plan. BP hopes to drill that well diagonally to intersect the original one below the seabed and then flood it with mud and concrete to stop the uncontrolled flow.

Although the idea sounds simple, the experience with a similar spill last year near Australia shows just how difficult it can be to execute the maneuver.

“It’s like finding a needle in a haystack,” said Rachel Siewert, an Australian senator who is a member of the country’s opposition Greens Party and is critical of the oil industry.

The Australian accident, known as the Montara spill, began Aug. 21 with a blowout of high-pressure oil similar to the one in the gulf. With the well spewing 17,000 to 85,000 gallons per day, precious weeks passed before the relief wells were started. When efforts got under way, the first four attempts — drilled on Oct. 6, 13, 17 and 24 — missed the original well.