perhaps this route is best as "topics/new". Think about it like this:

"new_topic" is in the top level directory. This means your top level directory has all these things related to topics. "topics/new" is inside the topics directory. When you "post" to topics, you're creating a new topic in side of the topics directory. When you "get" topics, you're listing all the topics in the direcyory. When you post to "topics/1" you're making a change to topic 1.

This is the foundation of REST.