If you've got space, perhaps a willow!

Uhuh. I'll think about that. Was hoping to plant an evergreen, though. Thanks for the reply.

Size might be a restriction, but the following sites have some suggestions. Tree fern would be my "go" if it's shady but it would be slow-growing. Bottle-brush (callistemon) or melaleuca would be a good choice, some of the latter grow in very swampy conditions.
I've put a direct link to the first one, which is
http://www.acs.edu.au/info/hobby/landscaping/wet-soil-plants.aspx

AND - wet area plants
http://www.eranurseries.com.au/category/68-wet-areas.aspx

Here is a link that might be useful: plants for wet places

Thanks Alisonoz for the links. I actually have a melaleuca (broad leaved paperbark) in a rather dry spot where it's not doing too well. I'm inspired by this information to move it to the wet spot and see how it goes. I'll post the result.

Jassy01, some melaleucas are suitable to dry spots and some to moist, also moving many Aussie natives is risky; they hate root disturbance. I'd do a bit of research on the one you have before I attempted moving it.

By the way j. if yours is m. quinquinerva (the broad-leafed paperbark) here's a link that might be helpful:
http://www.google.com.au/url?sa=t&rct=j&q=&esrc=s&frm=1&source=web&cd=2&cad=rja&ved=0CDoQFjAB&url=http%3A%2F%2Fwww.hornsby.nsw.gov.au%2Fmedia%2Fdocuments%2Fenvironment-and-waste%2Fbushland-and-biodiversity%2Fnative-tree-database-fact-sheets%2FFact-sheet-Melaleuca-quinquenervia-Broad-leaved-Paperbark.pdf&ei=p-pHUqz-BNCviQfGhIHICg&usg=AFQjCNG7s9atAT0G_woJcU4uin_JbISeUA&sig2=tQURHOWwsmOLGm2DRc-YJQ