Researchers have discovered a 120-metre long World War II Japanese "mega-submarine", missing since 1946, off the coast of Hawaii.

The I-400 Sen-Toku class submarine was captured and scuttled by the United States, along with four other submarines at the end of the war.

It was sunk off Oahu to avoid giving access to the technology on board to the Soviet Union, under the terms of a post-war treaty.

Terry Kerby from the Hawaii Undersea Research Laboratory (HURL) says researchers found the submarine in 700 metres of water of the south-west of Oahu.

"Finding it where we did was totally unexpected; all our research pointed to it being further out to sea," he said.

"The multi-beam anomalies that appear on a bottom survey chart can be anything from wrecks to rocks - you don't know until you go there.

"It was a thrill when the view of a giant submarine appeared out of the darkness."

The I-400 was the largest submarine ever built until nuclear-powered submarines were introduced in the 1960s

It had a range of 60,000 kilometres and was designed to carry and launch three plane bombers on the US mainland, but the ships saw only limited service before Japan surrendered in 1945.

Dr James Delgado from the National Oceanic and Atmospheric Administration's maritime heritage research team says until the I-400, submarines were used mainly for sinking surface ships or other submarines.

"The I-400 is technologically significant due to the design features associated with its large watertight hangar," he said.

"Following World War II, submarine experimentation and design changes would continue in this direction, eventually leading to ballistic missile launching capabilities for US submarines at the advent of the nuclear era."

The HURL crew identified the submarine wreckage in August and its discovery has now been confirmed after consultations with the US state department and the Japanese government.