One of the nicest looking restaurant venues I've found in Perth is Tsunami Sushi. No dated '80s interiors - it's all sleek and magical. Their food is pretty awesome too (expert tip: squid ink risotto makes your lips black. Use napkin often). The wedding packages are kind of French-Japanese fusion (as is much of their menu), so it shouldn't be too weird for Aunt Beryl and her strictly anglo taste buds. An ex-wedding photographer acquaintance rated it as one of the best metro reception locations she'd been to.
They're in Mosman Park, in a small strip of shops, so plenty of free parking about and not too far for your guests to travel from a ceremony in Freo or King's Park.