My guess it would be a state or federal contractual minimum that RFDS accepted
Wouldn't it be incumbent on RDFS management to advise whoever writes the state or federal statement of desired qualifications, that 650 hours instrument flight time is an unnecessary restriction since it cannot be audited by CASA. Same with specifying a set number of instrument rating renewals. Another leg pulling exercise.
There are often many cases where Australian pilots have been flying overseas for many years using foreign state of origin instrument ratings. Could be that they only have two or three Australian issued instrument rating renewals. That does not mean they are less experienced. In fact it is possible they have flown much more in bad weather than seen in OZ.
Stipulating X number of instrument flight time and X number of instrument rating renewals just shows how out of touch with the real world of flying qualifications that some Australian operators are.