SEOUL, South Korea — North Korea said on Saturday that Alek Sigley, the Australian student whom it deported this past week, had been a spy who admitted to “systematically” collecting information about the isolated country, with a state news agency reporting that he had been “caught red-handed.”

Mr. Sigley, 29, a graduate student in Korean literature at Kim Il-sung University in Pyongyang, was freed in North Korea on Thursday and deported on the same day. Until now, neither Mr. Sigley nor the North’s government had publicly explained why he had been detained.

On Saturday, North Korea’s official Korean Central News Agency, using the initials for the North’s official name, the Democratic People’s Republic of Korea, said Mr. Sigley was caught on Tuesday while “committing anti-D.P.R.K. incitement through the internet.”

“He honestly admitted his spying acts of systematically collecting and offering data about the domestic situation of the D.P.R.K. and repeatedly asked for pardon, apologizing for encroachment upon the sovereignty of the D.P.R.K.,” the news agency said. North Korea expelled him with “humanitarian leniency,” it said.