I haven't done any walking in the Blue Mountains/coastal areas of NSW.
Most of my walking has been in SA, NT, Victoria and Tas.
In SA I never take a water filter...most of the areas and waterholes are too remote, and I am yet to hear of any contamination of Giardia or anything. Even water with dead goats or roos in it is fine to drink in my experience. May not taste that great.
Same deal in NT. In Victoria I've filtered water a couple of times-we once took an old PUR Scout filter because it was dry-and it came in handy to get rid of the mud!
Tassie I have never done any water treatment-most of my walking there has been in either the SW (there are some lakes you dont take water, in case of "fecal" contamination-but otherwise pretty much any water is great) or in Cradle Mtn-Lake St Clair and Walls of Jerusalem; where there are tanks when there are regular people, and where people don't go, the lakes and streams are generally fine, with care.
I have heard in NSW, particularly in many rivers and canyons of the blue mountains, its not a good idea to drink the water without some sort of treatment, as there are towns up stream in the catchments. I know some experienced canyoners(albeit they are very conservative traditionalists) from there that have fallen sick just through accidentally getting water in their mouths.
It begs the question whether it is biological, agricultural or industrial contamination, or a combination. If its the first, and the water is clear, you will get away with simple chemical treatment. If its the latter two, and by this I am talking toxic heavy metals, pesticides, herbicides, dioxins, etc, then you will need to filter it if you are going to make it drinkable. And the only portable filtration I know of that will do it, is something with carbon in it-mind you if I new the water was heavily contaminated, I don't think I'd trust it. I think the Carbon is working by secondary chemical bonds (as far as I am aware) to adhere the toxins to it. Over time the Carbon will approach its bonding capacity, and if the load is heavy, some will get through.
I myself would probably filter water, but it depends on the location. If the headwaters of the source contain towns, industry, houses, agricultural areas...then I definitely would.
I am looking at doing an extended expedition in the NW arm of the Flinders Ranges in about august next year. This is a new water situation for me...sheep and cattle country. While I've worked alot in pastoral areas, I've never had to take water not directly from a Bore or tank that isnt already clean. All the water holes are visited by said animals, and are dams, and depending on the water level and source of the dams, etc, it could be interesting. I don't want to take a full filter. I am thinking of making a waterbag type filter with a coffee filter and mesh at the end, and put some clean, dry sand in on top...to filter out most of the muck, clay particles, etc. Then I will add some chemical treatment...likely Katadyn Puritabs, to kill any potential biological nasties.
Roger Caffin is probably the person to ask as to what he uses for water treatment, if anything, as he hikes alot in the Blue Mountains as far as I am aware.
Is it possible Roger to get chemical treatments such as Aqua Mira in Australia? I wouldn't mind something that works a bit faster than puritabs.
Hope this helps Andrew,
PS, how many Australians are on BPL? I think there may be about 5 or 6 of us in total. (You, me, Roger, a couple of others).