It actually has fallen below the Fed Funds rate, between 1997 and 2001 or during the dot.com bubble. One of the reasons i charted it was to see whether it could or it had.
But it should be noted that the level at which it fell to was far higher than what it is today. So the question is if the fed rate went higher from here can the Cash rate go lower or stay at these lows and for how long. The other thing to keep in mind is, this is the overnight cash rate not retail mortgage rate/s.
