Democratic Party representative Lee Cheol-hee reportedly cited unidentified defence officials as saying the hackers stole the plans past year.
One of the documents was a plan to assassinate North Korea's leader Kim Jong-un and other senior members of the regime in Pyongyang. He said that 235 gigabytes worth of military documents were taken with the content of almost 80 percent of them yet to be identified.
"We are in the process of upping the ante and increasing pressure on North Korea to change its behavior", Matthew J. Matthews, deputy assistant secretary of state for East Asian and Pacific affairs, told reporters last week.
-South Korean plans for a "decapitation" strike against the North Korean regime, a plan that has supposedly angered North Korean dictator Kim Jong Un.
South Korea's Unification Ministry has said safety guarantees for South Koreans are among the changes needed before it will consider restarting the tours.
Pyongyang has repeatedly denied any responsibility for or knowledge of the attacks.
Pyongyang conducted its fifth nuclear test on the anniversary of the founding day of North Korea a year ago.
Commenting on the news is Chris Doman, security researcher at AlienVault, who is investigating hacking groups in North Korea.
North Korea often tests nukes or rockets on key state anniversaries.
According to the South Korean officials, the drills last month took place at a training site which includes a model building of the Combined Forces Command.
The revelation comes amid heightened tensions over North Korea's nuclear and ballistic missile tests.
South Korea's Yonhap News reports that the Trump administration sent an advance team around the country in September to scout possible locations for Trump's visit.