I would Honestly just write them a letter saying why you didn't vote (busy/weren't aware/Don't watch anything but ABC2) and if there was a please explain letter you didn't get it and would assume it was due to a mess up with mail & moving.
For those saying how could she not Know... I was in Adelaide on holidays when I found out I wouldn't be in Sydney to vote on Sept 8th. I actually called the AEC and Mentioned that I don't watch anything but not news cable, I don't read newspapers and I don't socialise as I care for my husband and mother.. I got them & myself marked as unable to vote due to travelling.
They are pretty lenient on it really.