estates like you mentioned jimbo are a good idea in concept, however, they just dont work like you think they do.
i work in the water industry, and know for a fact that in australia, 90% of the time, the 'recycled' water that is used in new estates is drinking water from the main in the street.
why?
well because for a starter, there is no on site black water treatment facilities. it just cant be done for many reasons. on site recycled grey water (used water - but no human wastes) will only come from your basin and sinks, and possibly your dishwasher and washing machine. in the grand scheme of things, that isnt a lot of water. these onsite tanks not only rely on re-using water from the home, they rely heavily on rain water which is something we just dont have. you would need to wash your hands maybe 10 times before you had enough water to flush a toilet. the dishwasher and washing machine are another story though obviously using much more water per wash. when these tanks arent full enough, they are replenished with a 'make up water' supply from the town main.
black water (water used in the toilet) still goes to the sewer mains and on its way to the sewage treatment plant. so that is water gone. say the average family of 4 use the toilet 3 times a day each on average, thats about 100 litres a day going back to the sewage treatment plants..
i personally worked on a green building in melbourne - owned by the australian conservation foundation (acf) - they had all these fantastic ideas, and they deserve a lot of kudos for their ideas, but their green building, even after using the most water efficient fixtures, recycling their grey water - even treating their own black water on site, use more water per day than they can recycle. admittedly they save a lot of water, but they still use a lot of water, not to mention they treat their own black water on site which you could not do at home.
although off the topic of fancy green housing estates and buildings, i guess what you really need to do is look at that for a whole city.. 3.5 million people in melbourne, using on average 20 litres of unclaimable (on site at least) black water per day - thats 70,000,000 litres per day.
in my opinion, although re-using grey water is a great initiative and will help conserve water, but it requires people to want to spend their own hard earned cash to maintain their way of life when it comes to water use, its not just purchasing a tank, its running a lot of new pipework throughout the property. not everyone will want to or simply not be able to afford the money to setup grey water re-use at home.
the solution isnt as simple as putting recycled/desalinised water in our dams, there is a lot involved. educating the public.. convincing people to install grey water re-use systems to help conserve the water.. they're all big hurdles but i think they all have to be done.