India is investigating damage caused to French Scorpene submarines that are being built in the country, the defence ministry said on Wednesday, after the leak of documents relating to the vessel's combat capabilities.

The leak, which was first reported in The Australian newspaper, contains more than 22,000 pages outlining the secret capabilities of six submarines that French builder DCNS has designed for the Indian Navy.

‘I understand there has been a case of hacking,’ Indian Defence Minister Manohar Parrikar told reporters. ‘We will find out what has happened.’

The submarines are being built at a state-run shipyard in Mumbai and the first one was expected to join service by the end of the year, the first step in the navy's effort to rebuild its dwindling fleet.

The massive leak has also raised doubts about the security of DCNS's submarine project in Australia where it won a A$50 billion ($38.06 billion) contract to build the next generation of submarines.

DCNS beat out Germany's ThyssenKrupp AG and a Japanese-government backed bid by Mitsubishi Heavy Industries and Kawasaki Heavy Industries, in a blow to Japanese Prime Minister Shinzo Abe's push to develop defence export capabilities as part of a more muscular security agenda.

The leaked documents cover the Scorpene-class model and do not contain any details of the vessel currently being designed for the Australian fleet.

‘As a serious matter pertaining to the Indian Scorpene programme, French national authorities for defence security will formally investigate and determine the exact nature of the leaked documents,’ a DCNS spokeswoman said in a statement.

‘The matters in connection to India have no bearing on the Australian submarine programme which operates under the Australian government's arrangements for the protection of sensitive data.’

A spokesman for the French embassy in Canberra declined to comment when reached by Reuters.

MAJOR STRATEGIC PROBLEM

The breadth of detail in the documents creates a major strategic problem for India, Malaysia and Chile, all of which operate the same submarine, an Australian political source with decades of experience in the global arms industry told Reuters.

Excerpts published in redacted form on the newspaper's website contained highly sensitive details of the submarine including technical manuals and models of the boat's antennae.

‘If it's 22,400 pages, it's a major stuff-up,’ the source said. ‘It's a huge deal.

‘It allows them to understand everything about the submarines. What speeds it can do; how noisy it is; what speeds the mast can be raised at ... all of that is just devastating.’

The Indian Defence Ministry said it was probing the impact of the leak on the submarine programme which it said had occurred from abroad. It gave no details.

‘The available information is being examined at Integrated Headquarters, Ministry of Defence (Navy) and an analysis is being carried out by the concerned specialists,’ it said in a statement.

‘It appears that the source of leak is from overseas and not in India.’

Uday Bhaskar, a former naval officer, said that if the leak was established, it would amount to a significant compromise of the credibility of the submarines.

India has a fleet of 13 ageing submarines, only half of which are operational at any time, opening up a major gap with China which is expanding its maritime presence in the Indian Ocean.

Australian Prime Minister Malcolm Turnbull sought to deflect concerns about the leak, touting the high security standards in Australia, where the submarine will be built. The Australian reported that the leak occurred in France in 2011.

‘But clearly, it is a reminder that, particularly in this digital world, cyber security is of critical importance,’ he told the Seven TV network.