Well, yes, if the Qantas board stated, they didn't know about it, it is certainly odd.
The director concerned had a duty to report the the board "post haste" and perhaps it qualifies for ASX reporting as well.
I understand presumption of innocence etc and I know it is a trial by media.
She obviously doesn't want attention brought to the Qantas brand which is very admirable given the presumption.
She obviously decided to resign in the best interests of the brand because, it wasn't unknown to her that she was under an investigation, going off the previous newspaper articles - it was only a problem once the story was broken by a newspaper.