Not sure of the Reference (I can never find the proper reference using the AIP online), but it's 1600hrs UTC on the date before the one nominated as the Effective Date.
Eg. if it's effective date is the 12th of March, it becomes effective at 1600hrs UTC on the 11th of March (which is 0200 Local, AEST on the 12th).
Now even I will admit that I had to look that one up,
.
morno