Recieved letter saying I had to prove I had voted?

Hi all, me and my wife received a letter today from the AEC to say we apparently haven't voted in the recent council election in Victoria. But we did vote, via postal vote. We distinctly remember, and we discussed it for weeks prior.

We have to prove that we voted. I was thinking of writing a stat Dec, would that be sufficient?

Surely they'd wouldn't be able to definitively say whether or not our votes got lost in the mail? They're just a standard untracked letter.