Today Nintendo posted on their Japanese website that around 160,000 Nintendo accounts have been illegally accessed. A translation has been provided by @Sephazon. Nintendo warns that for the compromised accounts user’s nicknames, e-mail addresses, date of birth, and region may have been viewed by unauthorized users. However, Nintendo confirms Credit card information remained secure. To resolve this issue Nintendo has stopped allowing Nintendo account log-in via Nintendo IDs and they have reset all the passwords of accounts that may have been compromised. They will also offer refunds on any unauthorized purchases made on compromised accounts.
Nintendo’s announcement also recommends that all users activate 2-factor authorization on their Nintendo accounts to increase security.