Illegal residents are subject to fines ranging from KRW 1,000,000 for illegal stays of less than one month to KRW 20,000,000 for illegal stays of three years or more. The maximum penalty for employers of illegal residents is KRW 20,000,000 OR three years' imprisonment.

If you were deported as you were caught by the authorities, you would be banned from returning for up to 5 years regardless of the period of overstaying. ▸ There would be enhanced criminal penalty including accusation, etc., and increased fines would be imposed as well.