I do not know if Giardia is a problem in Tasmania but I have read about problems in some areas of the mainland.
It is certainly considered a problem in the US, this link to an article about Giardia was posted on a US based forum
http://www.bplite.com last night
http://erikschlimmer.com/pdf/GiardiaMyth-Buster.pdf and I thought it might be interesting to some here on BW-T.
I have never used any form of water treatment while out walking and never had a problem.
Tony