How is the decision made to deport someone?

When someone on a work permit is convicted of a crime here, why don’t they get automatically deported after they complete their sentence? I have read articles on expats committing crimes where they are ordered to be deported once they get out of prison and others where that doesn’t seem to be the case. What is the policy on this and which department is responsible for the decision on deportation?

Read Auntie’s answer and comment on CNS Local Life