FWIW a car with a roadworthy certificate can still be defected on the same day it received one, not just because many people change back to their defectable items as soon as their car has been cleared, but also because obtaining a RWC doesn't guarantee you against all defects - it simply states that your car was observed to be in a roadworthy condition by a workshop licensed by the RTA/VicRoads. If they missed an item or you got defected again for an item that was cleared, you could take it up with the workshop and VicRoads I guess, but it doesn't mean the car can't be defected.