It currently sits at about 30% give or take, which is extremely high given they are a developed country and compared to stats in the US and Japan of 10-15%.
Source: South Korea's Ministry of Health, via Reuters news agency:
An official at South Korea’s Ministry of Health said more than 30 percent of people who suffered fatal trauma injuries last year could have survived if they had access to proper, timely treatment. That’s far higher than the 10 to 15 percent in places such as the United States and Japan.