If you had an overseas bank account which is more than a billion won(equivalent around 800kU$) as of the end of 2017, you should have report it in the last June, otherwise you may got some penalty 10-20% of the bank balance.
1. Please check your resident status; it's more than 5 years during the last 10 years?
2. Please check your foreign bank balances: it's balance was more than 1bill.won even for just a one day during a year?
According to the "Adjustment of International tax act" article 34-37, a resident need to report it every following year in June including bank information, balance amount and so on.
Penalty is huge, so just report it first by the dead line and you can correct it afterwards if it's not that exact info.
For more details, please send me an email to firstname.lastname@example.org