The stolen documents included South Korean-US joint operational plans for a war with Kim Jong-un.
Former think-tank president Kim Tae-woo said: “This is a really serious matter.”
“Given the military is playing down this announcement, I am concerned at the extent of information that has been stolen by North Korea.”
A member of the South Korean government, Lee Cheol-hee, revealed the data theft which included the most up-to-date plans for war, Operational Plans 5015.
The highly centavo information was stolen by North Korean hackers in September 2016, according to reports.
Despotic leader Kim Jong-un is said to be enraged by the plans that include a so-called “decapitation strike” design to take out the country’s leadership.
Mr Lee said that 80 per cent of the stolen 235 gigabytes of documents have not yet been identified.
Key information on power plants and military facilities is also said to have been stolen.
The South Korean defence ministry had revealed their online network was infiltrated in May but did not reveal the nature of the hack.
Mr Lee said: “Part of my mind hopes the South Korean military intentionally leaked the classified documents to the North with the intention of having second strategy.”
The Hermit Kingdom is known to have focused resources on developing advance cyber-warfare capabilities.
Pyongyang hackers are suspected of attacking governments, banks and Sony. The film studio was reportedly targeted after creating a film about the assassination of Kim Jong-un.
Hackers have been accused of trying to steal Bitcoin from exchanges based on servers in South Korea as they secretive dictatorship seeks to lessen the financial pain caused by UN sanctions.
The hacking revelations come amid renewed tensions in South Korea as fears grow that Donald Trump will launch a military attack against their neighbour.
President Trump was accused of acting like a child and threatening to caused World War 3 wile negotiations continue.
The US President’s continued reckless threats towards world leaders could start a global conflict, Senator Bob Corker has said.
Mr Corker said: “He would have to concern anyone who cares about our nation.”
The Republican is locked in a heated feud with his President who he has accused of acting “like he’s doing ‘The Apprentice’ or something”.
The Tennessee lawmaker is concerned the President’s rhetoric could put the US “on the path to World War 3”.