SYDNEY (AFP) - A Japanese pearling ship which has lain on the ocean floor off northern Australia for more than 70 years will be protected for its historical significance, the government said on Sunday.

The Sanyo Maru, the only known wreck of a Japanese pearling mothership in Australian waters, will be enclosed within a 200-hectare protected zone, Environment and Heritage Minister Mark Butler said.

"We must ensure the protection of this unique shipwreck, an important part of Australian history," Mr Butler said.

"The lack of historical and contemporary information about the pearl shell trade in Australia reinforces the necessity of protecting the wreck of the Sanyo Maru and its relics as a significant part of Australian and Japanese shared maritime history."

The Sanyo Maru, which as mothership provided food, fuel and fresh water to pearling vessels, sank in July 1937 after it was hit by an unseasonal storm.

The boat was heavily loaded with harvested cargo and had 20 crew onboard.

All but two - a pearl diver and the ship's purser - survived.

Mr Butler said the wreck was in remarkable condition.

"The unique collection of relics at the site can provide us with unparalleled insight into the operation and technology of the pearl shell trade during the 1930s, and the day-to-day lives of the crews who sailed the ships and harvested the pearl shell," he added.

The boat sits on its keel at a depth of 27 metres, about 60 kilometres off the Central Arnhem Land coast.