I propose that all new and renovated elevated houses on a block size more than 500 square meters have a W bend (ie, two conventiontional U bends) on all bathtub and sink drains. The first one would need to have a screwed fitting on the low point. This would allow the homeowner to fairly easily fit a hose to the first u bend and drain the water onto the garden, and leave the second u bend as a water lock so you don't get smells from the sewer pipes coming out the plug hole. It would also allow the home owner to undo the changes very easily if there was too much water (ie heavy rain) or they didn't want the hassle of moving the hose, or they wanted to sell.

Never rely solely on grey water to water your garden,
as it will harm your garden.

You should always flush away the harmful nutrients
from your garden soil by applying clean tap water more
often than you use grey water.

Never use grey water on food plants of any sort:
IE veggies, herbs, fruit trees etc.

And don’t even consider using grey water near native
plants, as the phosphates from detergents will kill them
in no time.

—Burke's Backyard Fact Sheets, 2023.

I wonder if rain would also help.

The phosphates from soap are great for bananas.

They could do something similar with the downpipes from roof gutters.

Wasn't there some suburbs in a South Australian area that were purposely built and designed to have all their Grey Water from each property directed into a Wetlands catchment area to be 'filtered' and cleansed into purer (even drinkable, but for fear of exploitive lawsuits, this was not made official) water to be used for all other uses except 'drinking'?

Haven't heard of that one. If you get the government to do it, they will make it expensive and complicated. I've done it before with the bath water. I ran it onto the bananas. It worked well. Only problem was the lack of a water lock.