Best to use food safe plastic alot of people use rubbermaid brute garbage cans they can store plenty of water and are made with food safe plastic. Canada and the U.S use numbers 1-7 but it depends on country so I would google plastic food grade Australia to see of the system is different.
I use several containers to store my ro/di water.
One is a blue plastic 55 gallon container for general storage.
My ato container on my 233 gallon tank is a 32 gallon brute garbage can.
My ato container on my 66 gallon tank is a dog food storage container.
There are alot of different ways to do it.
eBay and Facebook Market Place always have used Food Grade Barrels for sale locally. Just make sure the label is still on them, for what they originally contained.
Not sure how much water you need to store at one time but I just use 5 gallon Sparkletts drinking water jugs (capped). *I have the service for drinking water anyway so empties are just available to me.