Citing information from unnamed defence officials, Democratic Party Representative Lee Cheol-hee said the hackers broke into the Defence Integrated Data Centre in September last year to steal the secret files, such as Operational Plans 5015 and 3100, Yonhap news agency reported.
OPLAN 5015 is the latest Seoul-Washington scheme to handle an all-out war with Pyongyang, which reportedly contains detailed procedures to "decapitate" the North Korean leadership. OPLAN 3100 is Seoul's plan to respond to the North's localised provocations.
Mr Lee said that 235 gigabytes of military documents were taken with the content of nearly 80 per cent of them yet to be identified.
Also, among them were contingency plans for the South's special forces, reports to allies' top commanders, and information on key military facilities and power plants, he added.
"The Ministry of National Defence has yet to find out about the content of 182 gigabytes of the total (stolen) data," Mr Lee said.
The hacking incident has raised calls for the allies to remain alert to the possibility of the belligerent regime readjusting its own contingency or wartime plans based on the stolen military documents.
In recent years, Seoul has been pushing to bolster its cyber defence capabilities as Pyongyang has launched a host of attacks on South Korean corporate and government websites by mobilising its specially trained personnel, including those based in China and other foreign countries.
Pyongyang has denied responsibility for the cyberattacks, including the latest one, upbraiding Seoul for "fabricating" claims about online attacks.