Today is a one-off national public holiday in Australia to mourn the loss of the Queen. I'd be curious to know when this attack started and whether it coincided with the public holiday by chance or by choice.
I don't know when it actually occurred, but usually this sort of announcement comes long after the incident. The announcement occurring on a holiday afternoon seems a little convenient.
That said, Optus knows they don't get in any real trouble for this sort of thing so they can only benefit from appearing to respond rapidly and transparently. (Which is a better PR move than being proactive)
New laws make it mandatory to report this major type of breach in Australia to the Australian Cyber Security Centre within 24 hours. They had no choice, or risk having the full weight of the government come at them for trying to cover it up.
That said, Optus knows they don't get in any real trouble for this sort of thing so they can only benefit from appearing to respond rapidly and transparently. (Which is a better PR move than being proactive)