This is the definitive answer on dead / dying cordylines that Bamboo wrote a couple of days ago. You are not alone - these plants have suffered all over the country and are on the sick list in gardens everywhere.

"Nothing you can do right now I'm afraid, its definitely a wait and see. First, check the trunks from top to bottom for soggy, oozing or rotting areas, particularly with orange gunk coming out. If the trunks are clear, then wait - you may find they put out new growth lower down the trunks, or from the base, or both, and then you can cut back to where the new growth appears. You may need to wait till June before new growth appears, if you can bear to. If you do have rotting areas, keep any eye on them - you may need to cut back beyond that point, but this is best left till April if at all possible. They are Cordyline australis, these plants, and they have been affected by the severe winter weather before Christmas. "

Good luck!