Why on earth your government is obliging you to have 15.000L water tank? it doesen't make any sense...
At times Australia can get very dry, and the dams supplying Sydney can become critically low, especially with the stupidly expanding population of Sydney. So there were water use restrictions, raised prices, public campaigns to make people save rainwater and use it for garden watering, etc.
But that was then, and recently the weather is much wetter. And will remain so for years, if Mr Maunder and his spotless Sun has anything to do with it. Currently all the dams are near full:
http://www.waternsw.com.au/supply/dam-levels/greater-sydneys-dam-levelsThen there's the AGW religious resistance to any suggestion of the climate becoming wetter for decades, because of course all climate change is baaaaad. So there's no rollback of the water use rules and pricing. Maunder glaciation will proceed faster than legislation change.
As for using it...
Firstly the brown stain is tannin, from gum leaves. In concentrated form it can be used to tan leather. I'd rather drink rainwater with tannin, than town water with chlorine and fluoride.
If you have somewhere on your property significantly higher than the house, you could put a second small tank there, with a small solar powered lift pump from the big tank. Then use gravity-fed water from the small tank for most house use, rather than town water. Showers, bath, toilets, etc. Filter for drinking and cooking - remove the tannin, still no fluorides.
Manual valve switchover to town water if there's been insufficient solar pumping and the small tank emptied. Keep it simple.
Insulating the main tank and using it as a thermal mass for house air-con is also a good idea.
Then there's the bushfire drench system. Something I plan to do when/if I move to the country. Not so much to save the house (mine would be fireproof anyway, partially underground), but to preserve surrounding trees. The idea is to spray drench everything in sight, before the fire front arrives, so nothing burns. Obviously, must be a diesel driven high capacity pump, since the town power would probably be out in such a situation.
If you already live in a moist fernery, there's probably little point to this.