Where does the magic number for Australia's refugee intake come from?

Australia commendably agreed this week to take an additional 12,000 refugees affected by the Syrian conflict. This almost doubles the humanitarian intake, from 13,750 to 25,750. Almost all the discussion about how many refugees Australia should or could take revolve around the figure of roughly 13,000. Why? How did this number come about? Why has it become the de facto starting point for debates about Australia’s response to refugees? And why the number 12,000 for the one-off intake of refugees displaced by the Syrian conflict?