Top comes out on the roof:
Not sure what happens below. Haven't gone under the house to inspect. Seems like a vent that travels through the wall space, but unclear why it would do that.
Any ideas what its purpose is?
To give an update:
I have shown it to a few asbestos removalists, and they all have said its an asbestos flume as many here have guessed. Can't really reuse it, will have to find another way to vent my roof space.
Thanks all for your help!