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?

Fireplace chimney.

Looks like a furnace exhaust.

This could be a few things, but based on the shape, I agree that it's most likely a chimney. It could also be a vent for something like a bathroom or a stovetop, but it's most likely a chimney for a fireplace/stove/etc.

(If you have a fireplace, remember to clean your chimney now and then as well.)

I don't have a fireplace, so that would be a surprise. It seems kinda small as well? And its relatively far from any bathrooms.

Don't have a furnace anymore, but when I did, it was entirely outside, on the other side of the house

If you don't have a fireplace, my guess is this is the flume of your stove or a furnace exhaust.

To add, it could also be something that's no longer in use. For example, I have family that removed a fireplace, but left the chimney intact. It made more sense to keep it there, but the fireplace was in a poor location, and they wanted it removed.

In my case, I have a wood-burning stove in the middle of my living room that's super annoying. I mean in the middle of the living room (where you'd normally put a couch). Whoever designed this is not human. If I could have it removed, I would, but the HOA would most likely not allow it :/

Furnace exhaust would be my next guess though.

Furnace exhaust would be ideal, because I don't have a furnace anymore and I do need a vent. I guess it depends if its safe to remove, I am a bit concerned it may be asbestos?

Hard to tell the diameter but it could be the water heater exhaust if you have a gas water heater.

Water gas heater was outside, but has since been replaced. I'm guessing whatever it is/was, it isnt in use anymore.

Then I would probably look in the ceiling access and see if it is just a leftover vent and make sure that excess blowing wind rain isn’t making its way directly into the attic. Those caps will prevent most rain but not all.

If they don't prevent rain intrusion, wouldn't that be a seriously flawed design?

Most vents share that flaw but most people have no idea that they do. There’s no way they can prevent some blowing rain into them. Most of the time it doesn’t matter but if the vent is abandoned into the attic, what little bit of rain does get through it will dump it straight into the attic ceiling. I’ve seen this way too many times.

Vents are designed to be venting, and that airflow is enough to keep moisture levels down. Stop the ventilation, and it just becomes a covered hole in the roof.

Other things it could be:

• Dryer exhaust
• Sewage stack vent
• Air supply for secret underground bunker

1. Could be, but not anymore. Not sure if dryer exhausts are common in Aus?
2. Fairly sure it isnt, I know where that is, and its a much different pipe.
3. I hope so, because that would be much cooler.

Ah, good point. If it's still functiomal it won't matter, but if it's just left open on the bottom it can be a problem.

Hadn't thought about that.

It probably is a left over vent, but I'm guessing it drops right into the wall space, and down under the house. My plan, if its not asbestos, is to reuse it for a bathroom vent, which would solve both issues.

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!