When you say she seems thirsty, what do you mean?
thats exactly what popped into my head when I read your post.
If you're concerned or have any you should speak to your ECHN.

Cooled boiled water is fine to give bubs, and they will take what they need - be careful though because even though its not formula, its still classed as part of their daily intake of fluids (ie:bubs might take less milk which would concern you however if theyve had a lot of water its still fine)

I personally wouldn't make it a regular thing though in case bubs begins to think it is a feed - they should get what they need from formula and breast milk.