If flying, a PLB is compulsory. It doesn't matter what else you have.
Very important you register it. Beacons - Australian Maritime Safety Authority
You can update and add lots of info. Fuel, range, pics, POB, telephone numbers, email, etc (whatever is easier to find and rescue you)
SPOT is a great device allowing designated others to follow you. I believe it is privately run out of the usa and they contact amsa when an SOS is activated. If you carried a PLB and SPOT, you would of course have your SPOT listed on your PLB registration.
The SPOT is not a replacement for a PLB.