You can go to the restroom, and top up an empty water bottle there, except it wont be cold, and if Sydney water, may be contaminated with 'acceptable' levels of giardia and cryptosporidium and who knows what. It will be 'fantastic' spreader if SAR's or Bird Flu , as less water= more coughs. Even the first class pax get to breathe the cattle class airbourne droplets. Translation: This restriction will cost you extra days off sick per year, and delayed flights.
Vancouver had water fountains installed, because someone had crunched the risk profiles, and medically, knows this saves lives and improves outcomes. They break the cycle, and lower risk considerably. Imagine USA citizen doing the same at his airport, and bringing West Nile into .au. Or return tourist from wherever bringing in Anopheles/Dengue. On arrival, they tip out their waterbottle on thirsty plants.... Customs and Quarrantine must be VERY worried. The zipper bags aso protect mozzies from insecticide, and keep germs moist and warm ...
Point blank, the airlines figure the average pax is packing 600gms of water, and are interested in closing the carry on luggage loophole so they can slug pax for excess luggage, while keeping the DF $180 million per year, that would be wiped out if duty free liquids were also banned (as they were when the eddict FIRST came out).
The REASON is to stop dangerous liquids. There is no reason why *prescribed* waterbottles could not be tested, other than cost, or as pointed out, a scam to manipulate profitability.
I will bring frozen ICE, Oranges, and bananas, or a whole watermelon to ensure liquid/electrolite balance is not impaired. Someone else can clean up the sticky mess, when 10 million passengers realise a bag of fruit is a good alternative to a water bottle, as pax stuff apple cores and banana skins, and pips, around their seats.