I'm planning on free camping at Thredbo for a week.
The girls are likely to run through the two water tanks fairly quickly, so I was planning on getting water from the river.
I don't plan to drink the water, but would like to filter it before putting into the tanks.
Any tips as to what to do?
Only thought I had was to pour the water through a cloth to catch sediments.
Do NOT drink the water in the Snowy Mountains it is NOT SAFE without extensive treatment. A filter of at least .1 micron which is a very very fine filter or boiling the water for at least 2 to3 minutes will render it safe. Effectively filtering the water to make it safe before putting it in the the tanks would not be practical as a.1 micron filter needs a lot of pressure to allow water to pass through and they are also not cheap.
Even above the snow line all the streams and bogs are contaminated with Giardia which is a very nasty bug. It is also very difficult to get rid of once it contaminates your water containers as it leaves tough inert spores behind even after the containers (and pipes) are drained and dried. The only way is to soak and flush the containers and pipes with dilute bleach very messy to do well and not miss as joints or tiny crevices.
http://en.wikipedia.org/wiki/Giardia
The water in the snowies got Giardia from tourists in the 1980s and it has spread to the highest places by animals who have caught it. The cure for Giardia is very nasty antibiotics, which I have taken on three occasions for Giardia, once from the snowies, once from contaminated water containers and another time from dodgy water tanks.
Best bet is take some water containers and fill them up in town.
Note: Diluted bleach from supermarket (generic brand is cheapest) is a good water purifier and tank and pipe cleaner. Be careful as it but tastes bad and is difficult to get concentration right and if you get it wrong (too dilute) you get sick.
Water purification tablets are simply food safe pool chlorine (and also is the same stuff as Napisan). Water purification tablets always taste dreadful. Bleach is a thing good to flush out water tanks and pipes if you are not sure of water quality, best to leave it in the container (and pipes) for 24hours or more then flush with safe clean water a couple of times. Do not leave in pumps etc for extended periods as corrosion from the chlorides in the bleach become an issue.