Having reached over KRW4 million in late May (well north of $3600 when Bitcoin in dollars was trading at around $2400), the premium for Bitcoin in Korea (via Korbit) has collapsed to zero...

It seems the gains in dollar-Bitcoin-exchanges vs non-dollar-Bitcoin-exchanges have helped the former in lieu of the latter (as arbs appears), and as CoinTelegraph notes the factors behind the extreme premium rates are starting to fade.

South Korea and Japan's extreme premium rates did not dematerialize overnight. It began with the stabilization of the Chinese market and the resumption of withdrawals led by the big three Bitcoin exchanges in China - Huobi, OKCoin and BTCC.

As the global market stabilized and the Chinese Bitcoin exchange market recovered, premiums started to decrease. Chinese exchanges, which used to process Bitcoin trades around 25 percent lower than the global average price, began to process trades at a value higher than the global average Bitcoin price.