Hi Sarah,
Unfortunately we don’t have information on percentage of buildings on Saibai with rainwater tanks. I could check and get back to you though.
Regarding the usage, rainwater tanks I’ve seen around are hooked up to various household uses such as toilet flushing, outdoor taps for watering gardens, washing clothes, and sometimes even for drinking water with a filtration system.
Hope that helps,
Cheers,
Sai