I am new to RORB and just wondering where do we go to get historical data for calibration?
Where you get historical data depends on where your catchment is. In general, you can get rainfall data from the Bureau of Meteorology and streamflow data from the water authority/department in charge of the area.
If you want rainfall data on a sub daily basis then you may need to pay a fee to get the rainfall data from the BoM, as for streamflow data, some state water departments provide streamflow data free of charge online but these are mainly located only on major rivers or tributaries. The local council might have their own rainfall or streamflow data but you would have to do your own research or contact them directly to find out.