I wouldn't do either, believe it or not rain water is not as pure as you would think and can actually have a lot of stuff (sometimes, very nasty stuff) mixed in with it. From all types of different chemicals to just plain ol' particulate matter. And the real kicker is you don't even know what could be in there! As conditions and the substances present in the air can change from day to day and even hour to hour.
And as for collecting from a downspout - it'll be even worse. Because not only will the rain water have whatever its collected from the clouds and the following journey down but also from the roof and following rain gutters. Not sure what type of roofing you have but here in the US asphalt shingles is very common. Asphalt is what a lot of roads are paved with and basically is what is left at the bottom of the pot after they refine crude oil
. Then generally most roofs have some tar and other chemicals, and most tin rain gutters are also coated with a few different compounds to protect them from the weather.
You'd be much safer if you can to use tap for the time being, make sure if you have treated water with chlorine you treat it appropriately, but an even better idea I'm not sure what it's like down under but here in the US a lot of our grocery stores have RO/DI machines. You simply bring a container of your own (or many sell containers for you) and you can get a few gallons of water for mere cents.