Building Commission Practice Note 2013-65 - Flood Hazard Areas and Building Regulations states that:
"Once a site has been determined to be in a flood hazard area, the new performance requirements will apply. The Australian Building Codes Board (ABCB) has developed a standard for the construction of buildings in flood hazard areas."
"The ABCB standard - Construction of Buildings in Flood Hazard Areas - applies where the flow rate of flood waters does not exceed a velocity of 1.5 m/s. The ABCB have advised that the reasons behind this rate are that typical construction methods are able to withstand up to 1.5 m/s and this rate is consistent with other international standards."
A copy of this standard is available on the ABCB website.
PC-Convey can be used to estimate flow velocities - see the Overbank Results or Table of Results screens.