The Red Cross has set up a database for people in affected areas to register to let family and friends know they're safe. The National Registration and Inquiry System database can also be used by those concerned about loved ones.
Go to https://emergency.redcross.org.au/
You can also call 1800 727 077 from within Australia. If you are calling from outside Australia, please call +61 3 9328 3991.
For other online emergency management sites, please see http://qldfloods.org/resource/websites-disaster-information