Cordyline australis drooping and yellowy

We bought a cordyline australis and planted it in a sunny position in my garden.  Two days later and after a cold night it was very droopy and some of the new leaves in the centre are yellow.  The place we bought it from thinks it will recover and needs watering (which we had done when planting) so I've watered it again. But no improvement as yet.  Any ideas?  Thanks for any help