Meccano:
Is it true that under Aussie Rules it is MANDATORY to carry an additional 1 hour holding fuel if there's a PROB30 CB's within an hour of ETA?
Not necessarily. From the Australian AIP:
"When thunderstorms or their associated severe turbulence or their probability is forecast at the destination, sufficient additional fuel must be carried to permit the aircraft to proceed to a suitable alternate or to hold for:
a. 30 minutes when the forecast is endorsed INTER; or
b. 60 minutes when the forecast is endorsed TEMPO."
The buffer is plus or minus 30 minutes of ETA.