There are a few places you can donate.

http://www.qld.gov.au/floods/donate.html - this is the government site to donate.
https://www.salvationarmy.com.au/new...FcuBpAodqQ6QIw - The Salvation Army is probably world known, but just in case it's not, it's basically an organisation that helps the poor and people less fortunate than others. They don't just help with money but also with clothes, food etc.

I'd say one of those is your best bet.