Sitting full of water, Dex or pink
plumbing stuff? Run in
salt or fresh water (might as well do the
raw water side as well).
The short answer is "yes, a dilute solution of muriatic acid", but to what degree depends on whether the fresh water circuit was stored properly, how cold it got and so on. I assume in your part of
Australia, frost isn't a problem?
While you're at it, examine the zincs for wear and check out our friend
Maine Sail's
photo essay on heat exchangers.
Westerbeke / Universal Marine Heat Exchanger Photo Gallery by Compass Marine at pbase.com
It may help you to interpret what you may find.