Need to know what part of the river you are interested in to even think of answering your query as there are big variations in velocity.
And these days not including flood influences is questionable, for it seems like flood events
are becoming ever more common.
Give us some more specificity and we'll try to help. I doubt if many of the folks manning the radio
at Iluka-Yamba VMR can give you quantitative velocities anyway... lots of the volunteers have little practical experience on the river.