how big do cordyline australis grow?

Hi Jules welcome to GoY. The simple answer is - not much more. There is a picture of my largest one by my pond in My photos. That is now about 15 feet and hasn't grown for a few years.

The only care I give it is tearing downwards (not cutting) the leaves off as they die off, and I think that is largely cosmetic care, it would probably survive anyway.

John.

Yours is not unusual by any means, but the healthiest/oldest in perfect conditions (hot and little wind to tear them) could well go to 18ft I believe. Mine reach about 15/16ft including flowers which look like boats and are filled with saccharin-sweet liquid for the humming birds. It's worth the effort to go in and pull out those huge thick flowering stems before they rot the base. Agree with Sarraceniac about the leaf removal.