When does the last continent become available?
 

The one southwest of Darion. I'm already level 49 for crying out loud!

Where are you in the story? You'll get to parts of that after you can go into the dwarven mines.

Okay, I got through to a new continent via the mines, there still seems to be an extra island under the shroud, so I'm assuming that must be the Riftlands?

Riftlands, Grotland, Hades again are all accessible at the same time via New Kordar tunnel (once you unlock one, all of them are open. Just check minimap to see the portals and mouse over. They tell you where they go).

Then there is Iceland and Tungsinn. (2 last maps).