Rhee Cheol-hee, a member of South Korea's ruling Democratic Party, said cyber attackers had stolen the top secret plans from the country's defence ministry database.
Among the stolen files was Operational Plan 5015 which contained up to date information on the North Korean leadership and plans to "decapitate" it, South Korean state news agency Yonhap reported.
Another dossier called Operational Plan 3100, which reportedly contains Seoul's planned response to any acts of aggression by the North around the Korean Peninsula, was also taken.
The leak is thought to include contingency plans for special forces and information about power plants and military bases in the South.
However, the extent of the breach is still unknown, as around 80 per cent of the contents of the 235 gigabytes of leaked data is yet to be identified, Mr Rhee said. This was despite the hack taking place in September 2016.
South Korea's defence ministry declined to comment on the fresh reports.
But state media in the country reported that its northern neighbours have targeted government networks and financial institutions in the country with a barrage of cyber attacks.
Security group FireEye recently accused North Korean hackers of trying to steal bitcoin from South Korean exchanges, as a way of mitigating financial pressures from punitive international sanctions.
The North has accused South Korea of "fabricating" the accusations.
Revelations about the contents of the documents came as tensions in the region remain high.
In recent months the pariah state has appeared to make significant steps in its development of nuclear weapons, testing what it claimed was a hydrogen bomb in September.
It has also launched a series of ballistic missiles over the Japanese mainland in recent months and threatened the Pacific island of Guam, a US territory.
US President Donald Trump has also traded a series bombastic threats and insults with the North Korean leader.
Over the weekend the US President hinted at military action remained on the agenda, tweeting that "only one thing will work" when dealing with isolated state.
Join our new commenting forum
Join thought-provoking conversations, follow other Independent readers and see their replies