A lot of new houses always have to worry about the extra spaces above ground water tank would occupied, if you put it underground it would cost a lot more.
While searching for information about Waffle Pod on Google, I found this company called Smef - Water Storage Innovation. They sell a water tank that is designed to be in the same dimension as a normal Waffle Pod. With this Smef thing, you can just install it like the waffle pod in your slab! How clever is that! No more expensive extra digging for underground tank, and no more huge ugly chemical factory tank (like our ne....), have a look at the photos below from their web site.
Not sure how much it cost though... Unfortunately looks like it's only available in VIC, and it's a bit too late for us. If you are in VIC, might want to give them a call!