The woman who started South Korea's #MeToo movement on Friday slammed the nation's highest court for quashing her abuser's conviction last month, saying the ruling effectively silenced whistleblowers and victims of sexual violence.

Seo Ji-hyun said she was groped by her superior Ahn Tae-geun at a funeral in 2010, and that he had her transferred from Seoul to a provincial position after she filed an internal complaint, blighting her career as a prosecutor.

She suffered in silence for years until she went public in a 2018 television interview, a rare move in a still conservative society where female victims of sexual assault are often reluctant to come forward for fear of shaming.

It triggered a flood of similar accusations against powerful men in fields ranging from art and literature to politics and religion that grew into a South Korean #MeToo movement.

Some top male South Korean figures including former presidential contender Ahn Hee-jung and once-celebrated theatre director Lee Yoon-taek have since been jailed.

But Seo's abuser, Ahn Tae-geun, walked free last month after his conviction for abuse of power was quashed by the country's top court, which ordered a retrial.

Ahn's release "practically paves the way for any companies or organisations to demote or fire internal whistle blowers as they please," Seo told reporters in Seoul on Friday.

It could be used to "discourage any potential... victims of workplace sexual assaults from coming forward and speaking truth", she said.

Seo, who has been on leave since 2018, is to join the justice ministry this year.

"I'm still afraid of retaliation, because my abuser used to be the most powerful figure within the prosecution and those who follow him still remain" influential in the organisation, she said.

Ahn -- who was separately fired for corruption in 2017 -- could not be charged with sex abuse because the one-year statute of limitations had expired.

Originally he had been jailed for two years for abuse of power in January last year, a verdict an appellate court upheld in July.

But the supreme court said it was difficult to conclude one of Ahn's actions -- asking a prosecutor to write a document related to Seo's transfer to a provincial post -- was a form of power abuse.

Some have called Seo a "gold digger" after she went public with her accusation, she said, adding prosecutors who tried to silence her had been promoted to higher posts.

But she had had the support of "many people to sow the seeds of change", she said.

"South Korean women will never go back to the past."