The Mining World and the Nether will both be reset tomorrow (Sunday 7th) at 6pm AEST. Make sure you grab all your belongings as anything left in these worlds will be deleted and will not be refunded. There is also a global broadcast in game to let everyone know what is happening.
Resets of these worlds will happen within the first week of a new month. There will be an announcement and news post before it happens.
UPDATE: These worlds have now been reset and are ready for use