South Korea's ambassador to Australia has resigned, Seoul's foreign ministry said Friday, after his appointment while under investigation by corruption authorities triggered public outcry just weeks before a general election.

Before being named to the post, Lee Jong-sup had been banned from traveling while South Korea's Corruption Investigation Office (CIO) probed allegations he interfered with an inquiry into the death of a marine last year, while he was the country's defence minister.

Seoul's foreign ministry told AFP Friday it had "decided to accept his resignation", without giving further details.

The 20-year-old marine died after being swept away while doing relief work during major flooding, with some reports saying he was never given a life jacket by authorities.

The CIO is looking into whether Lee, as defense chief, interfered with the probe into the marine's death. He has denied any wrongdoing and vowed to cooperate.

Lee was appointed ambassador to Australia by President Yoon Suk Yeol and left for Canberra on 10 March, triggering public outcry. 

He returned to Seoul last week, officially to attend defense meetings.

The ex-defence chief's posting to Australia was seen as an attempt by Seoul to enhance defense collaboration with Canberra, which signed a $2.4 billion deal in December with South Korea's Hanwha Aerospace to purchase infantry fighting vehicles.

But it also triggered widespread criticism, including from parliamentarians from Yoon's party, with the timing seen as unfortunate ahead of 10 April elections.

According to pollster Realmeter, "controversy over former minister Lee Jong-sup's appointment" has emerged as a variable in the run up to the vote, making it harder for the president to get his approval ratings up, the Yonhap News Agency reported.

Experts say the upcoming poll is crucial for Yoon's People Power Party (PPP), since the president could end up a lame duck for the final three years of his term if the opposition win in a landslide.

South Korea allows presidents to serve only a single term and Yoon's ends in 2027.

"It would be a disgrace to our country if (Lee Jong-sup) as a suspect, returns to Australia to engage in diplomatic activities," an editorial by local daily Kyunghyang Shinmun said prior to Lee's Friday resignation.

"The very issue began when a key suspect was appointed as the ambassador to Australia, which enabled his midnight escape-style departure in an effort to dodge negative repercussions on the general election," it added.