If you're not worried about cost, then yeah, a taxi from parra back to Darling Harbour would work. It'll probably cost the best part of $100 if not more. The ferry is a nice way to travel to and from the city to Parramatta if you've got plenty of time up your sleeve in the afternoon to make your way out west.
If the taxi fare above is too steep for your liking (as it would be for me), you could jump on a train from Parramatta, an express leaves Parramatta at 9:56pm and will have you at Central just after 10:20pm. Alight at Central for the light rail back to Darling Harbour... Total cost somewhere in the vicinity of $15 I'd imagine ..